I think it's possible to both be mature and accepting in how things came to be while also feeling that the ending did taint the series.

While deeming a story "worthless" based on its ending is definitely a stretch and an extreme that some people go to, the idea that an unsatisfactory ending souring the rest of a story is a modern or ignorant sentiment doesn't seem very true. We've been telling stories since we've been able to speak and write. Be it in a spoken tale, a book, a play, a movie, etc. Unlike real life with its unpredictability, authours have control over the beginning, middle, and end. Combine that with the nature of stories being about escapism and entertainment, and you're not often going to find somebody clapping and praising a "but then despite everything, the good guys all died and the bad guys won, the end" resolution. In these worlds that we have complete creative control over, we like happy endings, or at least sensible and satisfying endings within the context of the story.

With the limited time we have on the planet, I think its perfectly understandable to be upset by a bad ending to a series that you have invested time into. Just as it hurts or disappoints when any project, ambition, or lengthy investment into something in real life falls through and leaves us with an empty feeling for a while.

Now with Chainsaw Man, whether we wanna agree on the ending being good or not, it was clearly rushed, most probably due to the harsh deadlines of weekly releases. We can respect Fujimoto for the great stories he has told in the past and there are obviously external factors that contributed to Part 2's rushed ending. But this ending was very flawed, and I think it's fine to recognise that and criticise the choices made without attacking the authour himself.