It's not that it's the successor, it's that they can play it professionally and actually make money. If tf2 was monetized to the level Overwatch is, then they'd 100% move back, since it's a much deeper game, and more rewarding to play. Lots of tf2 players moved over and succeeded because it's a game that takes a lot of the same skills, in terms of teamwork and pushes (and to some extent, DM).
Move experienced tf2 players to any remotely similar team-based FPS and they're going to be pretty dominant.