Reddit was like this when it started. I was there when Dig was a competitor. It took a decade of constant posts by people who really loved their communities to really take off. And reddit, when it started, didnt have subreddits, it was closer to hackernews.
Not saying your arguments are wrong per-say. But it will take some time to get started. The good news is federation means different instances will come and go but the protocol means we dont need to stay on any one kind of server. Kinda like email. I use mastodon to talk with peertube creators and use lemmy to talk to other lemmy instances all the time.
multireddits
I think you are spot on here. We need this kind of functionality in order to keep up with instances. For example, lemmy.world/technology and lemmy.ml/technology, etc… could all bu there own multicommunities. Something like /mc/technologies or something saved on a per user perspective. It would transform lemmy overnight I think. multicommunities would make those niche topics have a great deal more action.