Given that Signal is free as a service, supporting federation only increases their expenses.
Element can do it for their Matrix servers. Process.one can do it for ejabberd. Prosody as well. Why can't Signal?
You've named several products that share your values. Perhaps those would be a better fit if you were to donate.
They've talked about this, a lot.
The internet would be a lot more efficient and able to evolve if we just had it controlled by one single entity like Google or Microsoft. Do you think is a good idea to do that?
The economy would be a lot more efficient and allocation of resources could be a lot more fair if we could put it all in the hands of one single corporation or government. Do you think it's a good idea to do that?
Agricultural output would improve significantly if all crops used the exact same genetic strain and if all soil was artificially managed. Do you think it's a good idea to do that?
In case you are wondering, "ability to quickly roll out post-quantum key exchange" is waaaaay down the list of my worries compared to "facing a catastrophic Black Swan affecting all of the world's communications".
There's plenty of diversity in the messaging space. Decide your values, choose your compromises, pick your platform. Simple.