It does make sense for Signal as this is a free app that does not make money from advertising. It makes money from donations.

So every single message, every single user, is a cost without any ongoing revenue to pay for it. You’re right about the long run but you’d need the cash up front to build out that infrastructure in the short term.

AWS is cheap in the sense that instead of an initial outlay for hardware, you largely only pay for actual use and can scale up and down easily as a result. The cost per user is probably going to be higher than if you were to completely self host long term, but that does then mean finding many millions to build and maintain data centres all around the world. Not attractive for an organisation living hand to mouth.

However what does not make sense is being so reliant on AWS. Using other providers to add more resilience to the network would make sense.

Unfortunately this comes back to the real issue - AWS is an example of a big tech company trying to dominate a market with cheap services now for a potential benefits of a long term monopoly and raised prices in the future. They have 30% market share and already an outage by Amazon is highly disruptive. Even at 30% we’re at the point of end users feeling locked in.