Whereas the web is the water that fills in the gaps for content, perhaps SMS is the water that fills in the gaps for communication and notifications.
In fact, when it's relegated to that role, all the constraints of SMS render it a great alternative to email and push notifications.
I've written recently on SMS continued decline but there is still smaller use cases that the now 20+ year old technology excels at. I'm still not sure it does anything better than push notifications (a simple example is the Guardian iOS app - swiping to the side gives the capability to 'share' or 'save for later reading' - all notifications can be bolstered with capabilities to suit their context which is not something SMS is capable of). Still though, I believe it has it's place in the near-term.
This coincides with the pioneer of SMS in it's current form passing away today. As he also highlighted in this interview (that took place over SMS!) - I've highlighted the important word in bold:
I believe that reliable convenient to use text messaging will stay forever. Is not necessary what we call sms.