I understand what you're saying but I don't know if calling it vendor lock in is quite correct. You can export your list of messages out of Signal and presumably import them to whenever you motivated to do so. Your friends could then join you on the new service and life would continue on.

I am personally unaware of any communication service with a universally portable user identifier and that allows you to freely switch upstream providers other than phone service. I think this what you're talking about?