Based on using the various solutions in this space as a player, text messages and mobile notifications are the norm (folks that don’t install the app will still need sms or similar).
I've integrated with Twilio. I pay a whole bunch of money every month for sending SMSs. And then recently they made me jump through a bunch of regulation hoops. Had to give them all my business documents.
So even if OP does all the work of integrating it, I assume OP isn't going to pay the fees for you, so then you have to create your own Twilio account and jump through the same hoops.
Not everyone does. It's a typical 80/20 problem (the 80% being those who use the technology that's easy to develop for, the other 20% being the hard part). 80/20 not being the exact numbers, obviously.
That being said, you certainly don't _have_ to cover more than one way of sending notifications, but that's a decision to be made and the consequences acknowledged.