Of course, they're mainstream artists but for Gaga, Pop is her actually genre.
All of the singles from '4' were aiming to be classic R&B sounds, Beyoncé doesn't really need Top 40 radio to be successful in this game. She is living in her prime at this point.
I think with Bey's 5th album, if she comes with a direction like IAMSF, Then Yeah! Her returning to "Top 40" isn't going to be a problem