While I would prefer both Discord and Telegram to have alternatives, they are popular for a reason. They are in experience what the Internet was in the 00s to the people actually using it a lot.
There are bigger platforms, which drive engagement and collect data as their business model, and they are convenient for everyone making decisions, except they don't solve any problems too well. Like a casino. It doesn't lead to anything good, but it's entertainment in itself.
TG and Discord are good at solving problems. They are Internet communication optimized - subscribe to a channel (or in Discord join a server, TG too has subchannels now, making a channel with subchannels similar to Discord) and say what you want to say, and read what you want to read.
Needless to say that this is pretty similar to IRC of old, and a reiteration of IRC with less load on servers, better security, structured messages, file transfers ... would perhaps be nicer, but a business model should be devised for such.