They’re a niche band if you compare them to pop acts today. But music in the 70s was different. Chicago, Yes, Genesis, Steely Dan, early Journey, and many others were popular bands in the 70s that were hugely popular and didn’t stick to traditional pop/baroque song structures and were technically advanced musicians. There are lots others that followed them into the 80s. In their time Rush was extremely popular and have maintained a following, and managed to bring in younger and new fans. I’d imagine if Yes and Rush both played today, Rush would probably bring in more people than Yes.