A few people can live with just phone calls, but a sizeable majority use some additional apps for texting. Dumb phones won't work here.
1. Many people use a virtual text number, like google voice
2. Maybe even more folks use one or more app based texting services. I bet many users here have several on their phone:
Signal
What's app
telegram
There are probably 50 texting type apps in this category
These are dangerous attack vectors for people trying to remotely control your phone, but also important to talk to your friends.
I think we need a solution for these types of apps for a popular usable solution. I don't know how to solve the safety issue when running these apps, and I can't just "forward" text messages to my dumb phone.
But most people including myself just don't care about side loading. For those who do, there are options like a Fairphone, various Linux phones, etc.