People don't use mms, they send stuff using their sms app, which decide to use an mms or not. In fact, long text messages are often turned into mms under the hood. People just don't know about it. As long as it works transparently, they don't have to.

Do people actually use SMS? Aren't they bloody expensive?

Sms, mms, and phone calls are included in a lot of cheap unlimited contracts accross europe.

It's free in a lot of places.