for your who cares question, sparse content feeds are one of the reasons small servers fail.
People can't find content on topics they are interested in, and they move to big servers instead, where those hashtag timelines have more content.
This is one of the reasons that 25% of Fediverse accounts are on mastodon.social.
In addition, not all Fediverse software has hashtag feeds, so people on other platforms don't have another way to follow topics they are interested in.
