The problem in this debate is between open source and open choice. Open source purist are often anti choice. If you want to use a closed source, proprietary system, that is your choice. The key aspect of federation isn't the open source (its great) but the open choice -- you can choose your own server (I have my own, FWIW) or some random tech bro or some evil Corp marketeer, or Meta, the point is choice. For each user, there are compelling arguments and compelling reasons for why someone would choose Lemmy or Threads or whatever, the value is in the choice.
So in the demand for our SABDFL to make a choice, we are in effect saying that we want to restrict the choice. Why do we care if there is a great community on Meta or that a great. Community on our server is attracting a broader community? Walled gardens are walled gardens. So I have to ask, what walled garden is the community asking for: open source and closed community? Sound like hell to me.