Although He-Man is a nostalgia thing, it doesn't mean that if there were a movie that came out that did well that it couldn't popularise it again.
For many in the general audience, they hadn't heard of the likes of Iron Man, Captain America, Thor, The Avengers and many others in the MCU. These were entirely brand new characters, even though they had been around for 50+ years. But with the films, this brought in an entirely new audience, and I'm sure brought many new fans even to these boards who had never read a comic, watched a cartoon or played with action figures in their lives before.
So Masters of the Universe could be a brand new franchise for audiences of today, but also a big nostalgia trip for those who were into it back in the 80s.