I actually disagree strongly with this analysis, and I want to detail some stuff here.
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.