To me, the argument for accepting Meta into the Fediverse goes beyond gain and loss. If you run an Internet service, you have a moral obligation to make a good faith attempt to interoperate with anyone using the protocol as intended.
By a similar token, if you run a mail server, you should accept SMTP connections as far as possible. Yes, you can ban spam, but you should not ban connections from Gmail even if Gmail is a privacy-destroying bad idea. By all means, allow individual users to set up their own block lists, but this should not be done at the server level.
Sure, just like you can run an SMTP server that blocks incoming connections from Gmail. It’s not illegal, obviously, but it goes against the spirit of an open, interoperable internet.