I agree with your analysis and feel all of it with increasing annoyance. I still use tumblr to babble about personal things (briefly and without a lot of detail, for various reasons), of course, but I really wish I could migrate back here to Dreamwidth... except none of my friends, both fandom and real life, will follow me.

I remember having accounts on LJ and DeviantArt and ff.net and a few other sites, and using mostly the same names for each, and 90% of my online (and plenty of offline) friends also having the same, and all of us interacting across all platforms as needed, and though the exact platforms might be different now, but it was really effective, both for fandom participation and for social interaction, and I miss having all those options.