You bring up this scenario, and my first thought is, I'm so glad I'm not in that alternate universe! I've listened to my parents' old albums, and I'm not impressed. You just can't feel the rhythm like you can with our music.
Of course, I've also listened to their old country albums, and it's not the same. I think what today is called country music is just that old rock music, with a little steel guitar or banjo or fiddle added.
If rock music dominated, with disco becoming a bad word, I can't see rap music developing its mainstream appeal. If people don't want to hear funky grooves in the music they're accustomed to, why would they be interested in people rapping over the same grooves?