Telegram is not a privacy-focused messenger. It is not a messenger at all, it is a dialog-based development platform, like WeChat. Most of its value is in public channels and groups, and bots. It is dead simple to build a useful Telegram bot; good luck doing this on Signal.

Wire and Threema are serious privacy-focused messengers. They have all things necessary for good privacy: full E2E, pseudonymous IDs not linked to phone numbers, multiple accounts, ephemeral accounts, side-channel authentication, etc.

Signal is something in between. It is not enough for state of the art privacy (as it is linked to phone numbers which are really tightly coupled to real-world identities), and it doesn’t provide a lot of features to be a good development platform. I think it is stuck in its current state, and no longer evolving.