Mainstream popularity.

If lemmy becomes big enough to have a user base that is a substantial slice of the population as a whole, corpo will try to ruin it as it ruined the internet as a whole.

So far the attention has mostly been a couple of content bots from news sites, and big socials spinning up their own instances only to get defenerated immediately. Once more effort is seen as profitable, that's when there will be a real problem with bots, ads camouflaged as content, astroturfing, and everything else that follows.