Decentralisation would just spread the costs

...the costs and the risks: let's jump forward a few years into financing issues, at what point does Signal become a liability and start operating against their stated mission, if the alternative is that they cannot survive? We are witnessing enough contemporary examples of enshittification to know that it's a real possibility, and that all centralized providers, but in particular the ones not charging for service, are at risk.

Some would even argue that this has already started in the case of Signal with their crypto payments and blocking of 3rd party clients which are clearly user-hostile.

Those individuals would have to collect contributions from their respective communities.

Perhaps, or perhaps not. Running costs get exponential with scale. You can host 1000 users on a shoebox computer/raspberry pi, but delivering a service for millions requires datacenter-level infrastructure and tons of engineering know-how.
Most people into self hosting or having a NAS at home can already accommodate their families, friends and more, which means millions of potential users, without the problem of trust from a single organization