In the absence of internet connection yea SMS is still useful but it has nowhere near the quality of UX and features of mobile messaging apps. It is just communication protocol after all.
Doesn’t apply from 4G onwards, where everything is IP, but still.
I recall the early SMS functionality being added to cellular phones after the introduction of 2G in the early 1990s and as I recall, one claim was it monetised the massive amount of spare signalling capacity on the network. In the UK I recall that being said in 1994 by Orange (the first UK 'challenger' network vs 1G incumbents CellNet and Vodafone) when I had a Nokia 2010 handset.
The main problem was too many phones in one place causing errors on the signaling line. Like a concert. You'd get to a point where maybe one call can go through every 10 seconds. Then one person tries to send an SMS "because he can't get through" ... and the phone helpfully keeps retrying to send the SMS, each retry causing >1 minute of nothing signaling on the network. The network is now dead and while existing calls work fine, you can't even hang up anymore, never mind making or receiving a call.