Wow, that New Posts chart is brutal. This nearby link[1] seems to claim that Yahoo failed to "not screw it up", but the details are extremely vague. I'm unclear what could have caused such a dramatic reversal. ~Everyone left, but why? What changed?
Personally, it's felt like a death of a thousand cuts. Tumblr was a gift to people who wanted one place to share "fandom" material whether it be fanart, fic, various graphics - basically a great place to land when livejournal/dreamwidth started going, and as someplace a little more personal and "warm" than twitter. There's been an ongoing harassment problem in recent years (similar to what twitter is facing) that didn't help, and tumblr's user base feels more anti-corp than most social media platforms, so the Yahoo buyout probably hurt it worse than usual. Pair that with the invasive advertisement "tweaks" (the algorithm-based feed was a nasty change for small creators like myself; it's not something that a third-party extension like xkit can get rid of easily) and the rise of alternatives like mastodon - and they've got a problem.
I still think tumblr has a lot of potential - it's still my favorite platform for the "fandom" scene and as an amazingly versatile platform for illustration/animation portfolios (ie a cleaner version of blogspot) - but I'm not optimistic these days.
So there were much fewer unique posts, and thus less reason/interest to invest time into making new posts.
At least this was my perception as a somewhat regular user among friends several years ago and why I ended up stopping using the platform.
Young people hate writing very much text.
Young people hate using posting interfaces of the sort Tumblr utilizes (it still comes across as a CMS posting system).
Young people hate too much effort toward customization, which is required to make a Tumblr page look half decent.
Enter Snapchat, WhatsApp, Kik, Viber, Instagram etc.
Click, record video, send.
Click, snap, add four words, send.
No layout customization bullshit. No concerns about something on your page breaking by mistake and having no idea how to fix it to get it back to normal. No diary/journal or blog text aspect, which most people have zero interest in.
It's too useful for what its newer users want (the ones that would be signing up and providing growth).
Simply put, Tumblr is a dinosaur. It happened that fast.
Tumblr was something that artists and fandoms took pride in putting time into. The grassroots were always the holder of the users despite everything mentioned here, even in 2010 and earlier. XKit tried its best to cater to the fans, but it felt as if it was being fought every step of the way by Tumblr itself. They had the power users that drove it, they will for some time still too, but again and again, they ignored the needs of their users, and the terribly broken comment system barely got a marginal fix just recently (past year?), which is still broken on individual blogs often. Blogs often got deleted for unknown reasons without warning and tons of users lost years of content and conglomeration of information.
I don't see Tumblr being in the same category as any of those you listed really except for maybe Instagram, where more professional artists are thriving. The culture was always the glue of Tumblr, and none of those services have it.
The CMS posting style was fine. The customization was great, and while retro, suited the community. There are tons of great text posts and educational content that was immensely helpful towards LGBT youth, sexual discovery, getting real history lessons to supplement the broken US high school history classes, art, music, fandoms, and so much more. Lots of it is still there even today. But it's a fringe community with very little marketing or ad value and a community that hates corporations more than any other group I can think of. There was never a path to monetization. I'm just sad to see it be killed through neglect of its true potential and actively pushing away users it felt like. I think its creator took a few wrong steps but in the end, realized there wasn't anything he could do with it and is hence moving on.