Aside from needing a phone, I really don’t see the issue with SMS. For sending quick messages to people directly, is there really a better tool? You (most likely) already have a phone plan, you already have a phone, and your phone is always on you. Everyone you’re trying to contact is in the same boat, even people who are technically illiterate. All phones with phone plans support SMS, unless there’s weird plans I don’t know about.
When it comes to more complex needs (group management, e2e encryption, etc) other services are better of course. I don’t use SMS to text my group of friends, it’s really not well suited for the job.
Aside from needing a phone
That is a huge one for me. Yes, I have a phone. But I spend most of my waking time in front of a desktop or laptop. With Whatsapp, Telegram or Signal I can read and write messages from the computer, copy/paste text to the documents I am writing, and send and download files. SMS are more limited.
SMS is hard to have on multiple devices and it is hell of a work to become SMS provider. Internet based messaging is better in a every single way.
Even the network range is now not an advantage, as 3G is shutting down in many regions.