Even if they have the source, they may not have all the build tools anymore.

Or they have the build tools but the wizard that set up the build system back in the day no longer works there.

Or they have the build system archived and documented but it doesn’t run because some license expired, and the tool vender doesn’t sell that version anymore.

In the near future, there will be another possibility - SaaS cloud tools that are impossible to preserve so they are forever lost.

What you’re describing is no longer federation but full P2P. From a purely technical point of view, it may work, but the biggest problem will be abuse (spam, excessive resource use, illegal content). When a new instance shows up, how do you know if it’s a spammer or not? And if an instance is blocked by another instance, whose side should you be on?