Federation significantly complicates usability, and Signal's approach tends to prioritize usability. This is probably its most significant difference from other open source crypto tools.
Your argument is that Signal should erase its differences and become like other open source crypto tools. But as you pointed out, the tool you're asking for already exists! If Matrix is already doing it right, then what's the problem? Surely everyone will switch over to the superior infrastructure and not look back.
I have a different explanation: Signal is successful specifically because of controversial decisions like refusing federation, and the reason other tools do not enjoy the same success is because of the usability compromises they have made.
Your argument is that Signal should erase its differences and become like other open source crypto tools. But as you pointed out, the tool you're asking for already exists! If Matrix is already doing it right, then what's the problem? Surely everyone will switch over to the superior infrastructure and not look back.
I have a different explanation: Signal is successful specifically because of controversial decisions like refusing federation, and the reason other tools do not enjoy the same success is because of the usability compromises they have made.