In order to avoid this, what would you think of having a "new joiners" instance, where
hexbear, lemmygrad and ml would be defederated
politics and news communities would be blocked at the instance level
That could help to onboard people, so that the first time they look around, they see more gardening, cute comics and casual conversation rather than another set of depressing memes.
Disclaimer: politics and societal issues are important and should be discussed extensively (they are quite popular on Lemmy, let's be honest). I'm not advocating to hide them all, just to not show them as the first content people potentially interested in Lemmy would see.
I see lots of at-least-weekly-active communities that aren't politics but also don't garner hundreds of upvotes. I'm not sure what the "dead" threshold for you is. Admittedly I also avoid political content like the plague and hide out in my little Subscribed-sometimes-Local hole, but the fact I can do this at all and come back to new posts every day means all the other communities are not dead. They just don't critical mass. Even without algorithms specifically tuned to push people to outrage bait, engagement bait, people still just naturally interact more with the outraging things.