The problem I see with instant messengers like Signal is the network effect: If Signal were to charge a monthly/per usage fee, I would be willing to pay that, but many people would not. With the walled garden ecosystem that are most messengers nowadays, paid messengers will have a much harder time gaining users and suffer the network effect.
If more open protocols like matrix were common, perhaps I could use a paid server that in exchange does not log my metadata. However I would be chatting mainly with people using free servers and these servers could be logging a lot of the metadata (basically everything except for my connection details such as client used and IP).