This argument just dismisses all criticism of the rules and implies that the "game" portion of the role-playing game is irrelevant. By that logic, the design of D&D 5e (and every single rule and mechanic in it) is no better or worse than any other game, including stuff like F.A.T.A.L.

If the rules don't matter, why bother? Why buy books, learn a whole system, and go through the effort of trying to use a specific RPG instead of just doing free form role-play?

If they do matter, then they can and will impact the quality of your experience in positive and negative ways. They can be well designed, easy to understand, and effective at serving their purpose, or they can be poorly designed, incomplete, confusing or nonfunctional.

Sure, you can ignore rules when you don't want to follow them, and you can do your own thing and homebrew it if you like. You can also ignore the ending of a book and write your own headcanon, that doesn't mean that there isn't any point in criticizing bad writing.

To put this another way, why have rule books and a character sheet with all those numbers on it? Why not just flip a coin whenever you want uncertainty about an outcome? Would a game with only that mechanic be just as effective as D&D at providing the type of experience that D&D is trying to create? If not, then why not? What makes the big complicated mess of rules that is D&D better than my single rule RPG?

This is a bad take. It would work if D&D 5e were the only rules in existence, but it isn't even the only version of D&D in the conversation, pretty alone the wife breadth of other systems out there. I've been singing Pathfinder 2e's praises for nearly 2 years now, and if the problem with PF is that it's too crunchy, there are numerous other much lighter systems out there like Dungeon World or 13th Age.

By all means, use 5e if it works for you, but that shouldn't stop criticism of it in places where the rules can be exploited, especially if other systems lack those exploits.

But it is useful for someone to make rules that are balanced and lead to interesting gameplay. There are entire companies that make their business around printing rule books and selling them to people who think it's worth the money.

Think of it like this: if you have a high enough GM skill, then you'll pass whatever check God has and make the game fun. But with more balanced and interesting rules, you get a bonus. Even if it's just a +1 bonus that makes your game 5% more likely to be fun, that means everyone who plays that system gets that bonus from one person making it. You'd be crazy not to do it.

Different things work for different groups. But some things tend to work well in general, and others do not.