I might if either of those had proper desktop clients (they don't). I spend most of my day sitting in front of a computer, so chat apps that offer only wonky web app "phone bridges" for desktop users don't make much sense for me.
The best part of iMessage and Telegram is that their desktop clients are as capable as their mobile clients, and they're real independent clients. Until WhatsApp and Signal offer that, I have little interest in them. Maybe the majority of the internet connected world conducts their communication primarily through a smartphone, but that's not me.