All of these quotes could have been used to argue against Assist Mode in Celeste, which lets players tune things like the game speed or jump total (or, perhaps, parry window) to their preference. But it had those and it was fine. You can *just not use them*. If the "easy mode" isn't as "good" as the regular mode (according to you), WHO CARES, the people playing easy mode weren't able to play normal mode at all, so by default it's better for them. (Also, I still don't understand how lengthening the parry window "breaks" the game. It simply makes a mechanic more forgiving. No one has been able to explain this to me in detail.)
I've never played Celeste so I'm not going to comment on whether I think the implementation of those features was good.
I think it's acceptable for a developer to care that the value of a game is diminished by the inclusion of difficulty modes that seem to undermine what makes the game good in the first place. I also think it's acceptable for a developer to not care, since as you've pointed out numerous times these features are very easy to avoid and they don't affect my experience at all (see: Bayonetta). My problem is not with games having multiple difficulties, it's the framing of the issue here as being exactly like games not including colorblindness features and saying that the developers are bad or wrong for not including multiple difficulties.
The problem with lengthening the parry window is that where it is right now was clearly tuned in such a way to get players gradually on board with what the game is trying to get you to do. The parry window is actually quite large, but the game makes you think it's small because it punishes certain behaviors harshly. This is a rather clever way of getting players to focus on getting their defense together, and then in the run-up to Genichiro the game gradually forces you to start to figure out how to work an offense into your plans.
If Sekiro's difficulty was really about the parry window, then the game wouldn't have been completed by nearly as many people as it was. Indeed, even on the last boss the game is pretty generous with the timing, and the challenge comes from the fact that you have to stay calm because Sekiro makes you commit to the action you picked. A great example of how this works is when you have to send back lightning. The timing window for this is
massive and you can basically mash it out at any point before the attack lands (Mikiri works like this too). The difficulty comes from the fact that if you panic and don't jump you'll just get hit. But it's still not really about timing.
Perhaps there's something to be said about Sekiro being relatively opaque about trying to get you to do what it wants. A lot of the players I know who were stonewalled by the game had problems because they didn't realize that they, say, had to swing back at their opponents or they were going to lose no matter how well they parried. Also, the game is filled to the brim with worthless mechanics that I could see players getting sidetracked by and then wondering why the game is impossible. But I digress: My point is simply that the parry window size isn't an accident and they clearly put a lot of work into setting it at a point where players with a poor reaction time
like me won't be completely shut out of the experience while still getting people to do the right stuff.
But certainly TGM3's easy mode is nothing for those crazy grandmasters, and the existence of an easier mode does not take away from the game's mystique or mythos. So if they offered an even easier option, it would still have all these mystique about it. The existence of any easier mode doesn't affect those at the top.
It lets you play the game's first 50 levels, and "sakura is whatever" is not a good counterargument. TGM3 still offers players of lesser skill some opportunities to enjoy the game, even if you think they aren't the "real experience". Could it use more? I would say yes. Also, even if it didn't, players can get a *very* comparable experience (and easier!) in other Tetris games, where they could not in Sekiro.
The bolded isn't really true. None of the best western players, for example, are particularly good at easy. It's really not a good example of a well-implemented "easy" mode; it is a very difficult mode with an inappropriate name that hides its difficulty a little better than master or shirase. I said "sakura is whatever" because sakura is clearly a minigame; its inclusion doesn't change the fact that the vast majority of the game is completely unplayable for most of the population. Also, its execution barriers are still pretty high so even if it were representative of the main game it's not really doing anything for your argument.
TGM3, by any objective measure, has no easy modes. The two modes that have more lenient clearing requirements are best characterized as minigames because they don't reflect the experience of playing the overall game (easy is about economy and making combos which don't play into the main modes at all and sakura is a puzzle game with completely different objectives/parameters).
Playing the first 50 levels of shirase means placing 50 pieces, clearing no lines and losing. This is akin to dying in the tutorial of Sekiro (or perhaps in the menu, all things considered).