It's less the idea of adult content ending on Tumblr and more that their auto flag system is so godawful that instead of coming to fix it they just decided to ban any NSFW content outright and don't check the false flags. It's damaging to "muh porn", sure, but it's worse because as a place that has a healthy community of people often younger and marginalized it was often used as a place to show experimental art (not just pornographic) and as a whole made accessing content and sharing content accessible. The NSFW side of things also stretches to people who do NSFW stuff for a living (art, fics, photos, etc) so to just up and say "we got tired of moderating the spambots so let's just kill it all off" it shows a profound misunderstanding of what the site is actually used for.
Combine with an overzealous bot that is flagging everything from memes of aliens to dumb text posts that say The F Word to images of Ana Amari from Overwatch (because Anorexia was shortened to Ana in tags often and said tag was deemed a "nsfw" tag despite being used for support for people with eating disorders) you have a recipe for disaster. I don't know if Tumblr will truly "die" but it's a shame that they're getting a huge portion of the community to abandon ship because the mods are too lazy to deal with the spambot problem.
To some people it might seem as simple as "ha ha good riddance" but in reality losing fandom spaces can be upsetting and dangerous, especially one as tied to the convention scene as Tumblr. It's like, damn, guess it's time to find another place. But that doesn't mean it's the same. To use Smashboards as an example, if Smashboards went down tomorrow for good, sure you could find a new forum but that is literal years of history, our history as fans, gone in the blink of an eye because of an arbitrary decision, and it doesn't mean that whatever rises up to replace it will ever be the same because who knows where everyone is migrating to?
And before anyone says "we're all dust in the wind bro I'm edgy and read catcher in the rye once" yeah I know that. Everyone does. But as humans we need a place to express ourselves and our identities and our histories in those identities, as weird as that is, so to lose a pillar of that community suddenly with two weeks of warning has everyone panicking just a little.