First, there is the statement that features such as the "Instant Level 99" button in the most recent Final Fantasy re-releases are optional, and that players can forego their use, which would leave the features having no impact on the experience. On a surface level, this is certainly true, but it must be pointed out that microtransactions in games like Dead Space 3 are also "optional" and that, on the surface, the existence of such features doesn't have any effect on players who pass them up.
On a deeper level, however, this isn't the case; it certainly has an impact. Resigning oneself to skip the White Tanooki Suit of Super Mario 3D World allegedly will have no impact on the experience by simply being there as an option. This is false. Nothing changes whether the player grabs the suit and cheeses through the level, or if the player foregoes the suit and struggles on, finally overcoming the challenge. Nothing changes, but in one scenario the player had an easy time, while in the other the player fought. There is no difference as Mario climbs the flagpole.
In actuality, Super Mario 3D World is a great example of how to do this right, and the preceding paragraph is wrong. Rosalina cannot be unlocked if previous levels were completed with the White Tanooki Suit. When the game is completed, a meaningless sheen will appear on the save file if the suit wasn't used, but that's negligible and far too inconsequential to weigh in. Being rewarded with a new character for passing up on using the suit, for persevering in the face of the challenges, and for struggling and overcoming is more than a sufficient reward for not using the "I Win Button."
Without Rosalina being sealed away from players who simply died repeatedly and 'cheated' their way through with the White Tanooki Suit, there would be no meaningful reward whatsoever for actually playing the game. If Rosalina was available regardless, then there would be no difference between fighting through the stages or simply running off the cliff repeatedly, grabbing the White Tanooki Suit, and simply charging through the level. Players who unlocked Rosalina would have essentially achieved exactly the same thing, which would undermine and erase any sense of accomplishment that would have otherwise come from having unlocked Rosalina.
This is critically important, because the sense of accomplishment is a big part of why we play games, and it's the reason that games like Dark Souls II and Dark Souls III are popular despite being extremely unfriendly toward new players - people want a sense of accomplishment. Two primary things factor into this sense of accomplishment: how easy it is to do, and how many people have done it.
There is no sense of accomplishment in successfully putting on socks in the morning, because that is an easy task, and anyone can do it, yet there is a great sense of accomplishment for those who have knitted their own socks. What is the difference? One is much easier than the other is, and one is done much more frequently than the other is.
In a way, the Invincibility Mode of Star Fox Zero is like a Participation Trophy. Participation trophies are fine, in theory, and aren't a problem in and of themselves. There is only an issue when excellence is not rewarded. When the player who hit seventeen homeruns in the last season received exactly the same award as the player who sat in the grass eating bugs, there is a problem.
Whether we like it or not, and whether it's politically correct or not, striving for excellence must be encouraged, and the best way to encourage people to work hard to improve is to reward excellence. Participation trophies and "I Win Buttons" are not a problem, and it's always good to encourage everyone, to include everyone, and to bolster the self-esteem of everyone involved, but this does not mean that excellence should go overlooked and unrewarded. Pulling away a child who received a participation trophy, pointing at the player who received the larger, shinier Home Run Trophy, and saying, "You see that? If you work hard, that can be you," would go infinitely further than the participation trophy alone.
Unlike many, I trust Nintendo to do the videogame equivalent of this - it did it in Super Mario 3D World, after all, and it has been doing it for thirty years. I remember beating Contra III: The Alien Wars on Normal and reading, in essence, "Congratulations, and good job! But… if you try harder, we have something else to give you." I had played the game, and I had enjoyed the game. Knowing that something else awaited me for completing the game on Hard encouraged me further to try again, to improve, to further enjoy the game, and to overcome the challenges.
When I do the Level 1 Challenge in Final Fantasy IX, I'm fully aware that bragging rights are the only reward, because that is the nature of self-imposed challenges. They are undertaken solely for one's own sense of accomplishment and it is expected that the game will neither notice nor care that the player is handicapped. However, the implementation of Invincibility Modes fundamentally changes the game as a whole, and it renders "simply playing the game normally" as a self-imposed challenge. Just playing becomes a self-imposed challenge and, as is the case with all self-imposed challenges, there is no real reason to undergo it but bragging rights. In order for someone to undertake such a task, however, the bragging rights must be substantial.
"Ooh, you beat Final Fantasy IX without using the Instant Level 99 feature. Woo-hoo."
Exactly. Final Fantasy IX isn't hard in the first place - most JRPGs aren't. When they are difficult, it's usually toward the very beginning of the game, leading to easier gameplay as progress is made. Why bother, then, to play through the Android re-release normally? The difficulty difference is negligible, so the self-imposed challenge offers no bragging rights, and anyone who wants a self-imposed challenge would likely go for the Level 1 Challenge. Why bother at all?
However, the Easy Mode itself is not a problem, nor is it a problem when it's taken to slippery slope degrees and turned into a "Can't Lose Mode." The existence of such a mode has no direct effect on the experience, after all. The issue, as stated, is the lack of rewards for those who choose not to use the mode. Self-imposed challenges aren't supposed to offer in-game rewards; it's in the name and is what makes them self-imposed. When we turn "simply playing the game normally" into a self-imposed challenge, though, something is wrong.
It's important to offer people rewards for doing better. In fact, there may be nothing on the planet more important. The reality faced is that videogames are a huge part of the culture, and their increasing ubiquity in the world means that they have profound and subtle influences on our behavior. It's a slippery slope to say that Doom causes people to go on killing sprees, but multitudes of evidence supports the notion that videogames do affect us in subtle ways.
A bass player in a former rock band I was in happened to have a vehicle that dinged obnoxiously and incessantly if the driver seat was occupied but the seatbelt wasn't fastened. After only a few weeks of owning the vehicle, his behavior had changed; before even cranking a vehicle, he would fasten the seatbelt. He had been trained, conditioned. A system of reward and punishment had affected him in the most subtle of ways: in order to stop the noise, he fastened his seatbelt. The punishment of hearing the noise for not fastening his seatbelt subconsciously trained him to always wear it. It's scary to think of and acknowledge, but we are all being conditioned like this, and being unaware of it makes it more likely to be successful.
The conclusion is inescapable: failing to reward those who strive for excellence can have the side effect of eliminating the desire to strive for excellence. Videogames have a responsibility to humanity to do what is right and what is best, and it is certainly in humanity's best interest to have people striving for excellence. The best way to achieve this is to offer rewards to those who do.