Right now, for some of us at least, it's kind of early days for watching subs only, my feed is kind of slow without being on All, and it helps me discover new communities.
This is exactly why I’m almost exclusively on All right now. If I only browsed my subscribed feed it would be, like you said, mostly dead.
I’m also using this as an opportunity to expand my horizons by trying communities I may not have seen or given a chance to on Reddit. It’s nice because overall Lemmy is a much nicer place to comment, so far, and I find myself being more willing to put my thoughts out there.