“I would say, though, that ‘pop’ simply means ‘popular’ - which means it could be anything. In truth, ‘pop’ means nothing musically”
Very true. For instance, one of the reasons they dubbed Michael Jackson the “king of pop,” aside from the fact that he was popular, is that he so mixed various styles—soul, r&b, funk, disco, rock—that it was impossible to tease them out and pin him down. I would argue, however, that at different times there are dominant styles. Therefore when you said “pop” in 1977 it more than anything else meant disco, and to say it today largely means hip-hop.
That’s not to say pop and hip-hop, or pop and disco, or pop and rocknroll, or pop and jazz, or pop and whatever will ever be synonymous.
Elvis was the king of pop.
Listen to the originals/predecessors of many of Elvis’ better known recordings and you’ll realize all of the genres he incorporated (Al Jolson’s recording of Are You Lonesome Tonight may be eye opening).
The Beatles held one foot in pop as well and certainly their catalog became one of the last groups to create such a song list that was successfully covered and reinterpreted by so many other bands.