However most of my instant messaging is done on a computer using discord, so I might not be in the prime user base of these apps
> I don't get it how having a dozen of messaging apps and remembering who uses what is better than a simple SMS
Indeed, especially now that Telegram is taking off by times in Europe (in Belgium / Spain / France at least Telegram is getting used by a lot of people) and that some people now refuse to use WhatsApp.
In addition to appointment reminders from doctor/dentist/notary/whatever and delivery tracking numbers I still exchange SMS with quite some people.
It's not as if it was exactly hard to open and reply to a SMS you just received from someone: takes exactly the same time as answering using WhatsApp or Telegram.
SMS is used by companies to send notifications and asking for confirmation, even (ouch!) banks. I haven't sent one in more than a decade.
Interesting, here in Germany, almost everyone I know has Signal and WhatsApp with some people using only one of them. Telegram I encountered from one US American living here, and from people into conspiracy theories.
As for Telegram, people mostly use it to consume news. It basically replaced RSS readers for common people. Although its install base is relatively high, I have yet to receive a single private message over Telegram.