Not the OP, but here are just a few things that immediately sprang to my mind:
* Group MMS messages are a nightmare so people in my demographic often use GroupMe to facilitate them
* I'm not even sure if attaching images and other media on MMS messages is supposed to work consistently. I usually end up with an image that has been severely degraded in quality
* No read notifications, which can be useful if you goose to enable them on WhatsApp, Signal, etc.
* Poor security, I would assume that SMS is far easier for law enforcement to illegally bug
* Conversing with people who are in different countries as SMS typically invokes outrageous charges, and my mobile provider includes 2G data in most countries
* Can't use SMS on a computer unless through something like Google Voice, which isn't a pure SMS solution
* Message history backups are much better documented on WhatsApp/Signal, so that I've never bothered to figure out how to restore SMS history after I get a new phone or install a new ROM on my phone (Android)