You're not wrong. Federation would have higher costs but distributed over more people. Even with pure P2P a-la BitTorrent things might not be significantly cheaper because you'd likely still need to host authentication centrally or federally. You'd only eliminate the message bandwidth costs.

The thing is, we already have a way to distribute the costs - people subscribe to support Signal. Some pay more, others less. Whether I run a node that serves 100 people or subscribe for $10/month, it's somewhat equivalent. So the practical takeaway should be - if you want for Signal to keep signalling - subscribe if you can afford it.

The difference is that there's enough unused capacity on your personal device to handle all the traffic any typical user needs to handle in a day many times over, for simple messaging. Likely, that load is so little it won't even affect your battery life.