With how incredibly cruel people can be in fandoms nowadays, I can understand the lack of interaction on tumblr.
I know it's well meaning, but I have a feeling folks who say "people who boop can also interact directly with blogs!" have never been sent hate mail, or received nasty comments, or been put on a list that gets passed around because they ship/write/draw the "wrong" thing.
SO yeah. The lack of interaction isn't what's killing fandoms. It's the people who go out of their way to harass other fans that leads to less interaction that kills fandoms.
And it's not really fair to expect people to interact in a hostile environment.