Tumblr's only major problem was policy and the drive for ever-more ad impressions to please investors. Automattic could pay off what they spent on Tumblr by passing a hat around at one of their meetups, and they eased up on the policy. A lot of people really liked Tumblr, and it fills a lot of gaps that nothing else really fits. Automattic has enough trust from their handling of WordPress that people are trickling back in.
Culturally, Tumblr solves the problem of blogs where it's hard not to feel like a post is wasted if it's not a big, serious article. Tiny posts, images, links, and goofs get around just as well as big, serious posts. It solves in one platform what the fediverse is trying to solve with an awkward amalgam of Write.as, Pixelfed, and Mastodon. It might get there with a few years of iterations, but Tumblr is there today if you can stand a centralized platform. They at least let you use your own domain, unlike Twitter.
I agree, a lot of popular Twitter folks are Tumblr refugees after the great purge ("female presenting nipples"). It was always the policy that ruined Tumblr, the concept has a lot of potential
Culturally, Tumblr solves the problem of blogs where it's hard not to feel like a post is wasted if it's not a big, serious article. Tiny posts, images, links, and goofs get around just as well as big, serious posts. It solves in one platform what the fediverse is trying to solve with an awkward amalgam of Write.as, Pixelfed, and Mastodon. It might get there with a few years of iterations, but Tumblr is there today if you can stand a centralized platform. They at least let you use your own domain, unlike Twitter.