A customer of mine develops a surprising number of web apps with Rails. REST APIs or full stack apps with Turbo/Hotwire/Stimulus whatever the name is. I've been working with them for six months and I saw and worked on at least five apps at different state of maintenence and delivery, including one started from scratch and completed in three months with a React front end. Maybe there are more productive frameworks but Rails is still a good way to make developers productive.

Are they SaaS apps intended to be used by millions of people? No, maybe only in the thousands or tens of thousands but I think that not many SaaS are used daily by million of people. There are only 24 hours in a day.