I don't think a "main" instance is something we want. If one instance completely takes over the whole federation, we will have another reddit debacle in a few years.
While Decentralization is not a silver bullet against monopoly (just look at what gmail did to e-mails), centralization seems to always kill independence once platforms reach a critical mass
Reminds me of the time used a private gitlab repo for a freelance contract where I was working alone. I used it to keep track of tasks in issues. Some issues in this repo really turned into me talking to myself