4. I think we underestimate the power billionaires can have when they want to take over an entire technology. If they really put their power and money to it, they don't need to "outcompete" us with their own services, they can go to the root and sabotage ActivityPub itself.
4/
And finally
5. There are a number of legal problems the fediverse is ripe for exploitation for. Copyright is a main issue - every server just violates copyright on the daily, on a mass scale, simply by including posts from other servers in their timeline that the individual user has not given consent to. Enterprising attorneys may discover this as a source of income in the not too distant future.
5/5