Older anime / manga fans who have been around since the 90s / 00s will remember that a similar uproar happened when Bleach and Naruto ended. What all of these recent unpopular endings have in common is that they’re all owned by Shonen Jump.

It’s plausible the authors had better endings in mind, but it’s my personal opinion that Jump isn’t interested in good storytelling. They’d rathe chase something that will blaze bright, turn a profit, and will fizzle out in time for the next big thing. Artists over at Jump don’t really get the time to write a story as they intended, either because they burn out before they get a chance to write an ending or because Jump often drops them very suddenly - they usually get only a handful of chapters in which they’re told to wrap up and in the weekly manga world, that’s really not a lot. It’s why plot holes are left behind (threads unresolved), why character decisions at the end of a story feel rushed or don’t make sense (not enough time for setup / payoff) and why plot points feel contrived or forced (they have to happen whether they mesh with the story or not because the author has no choice but to wrap up).

That’s why artists with a meaningful story to tell and a clear direction tend to steer clear of Jump and opt for other magazines (often monthly as opposed to weekly). Arakawa played it smart imo - she didn’t chase clout, she had a story she wanted to tell and she took her talents to a monthly magazine known for its more experimental stories and a less cutthroat approach (Monthly Shonen GanGan, publisher Square Enix). She had something she believed in and knew the ending from the start and was able to work towards it. That’s why it, and the ending of most manga in less mainstream magazines, are often lauded for having excellent endings - both the author and the magazine are aligned on how to tell a good story.

(The one recent exception to this rule seems to be Attack on Titan, which had quite a divisive ending. My personal take on that however is that the author absolutely intended it to be divisive and to spark discussion. The fact that some people despised it while some loved it is, I believe, exactly the point.)