Well, you gotta consider that TF2 hasn't been a good game for YEARS now.
https://www.youtube.com/watch?v=Q9RjDRLBWio
Overwatch is the first ray of light in at least half a decade.
I've been a big critic of what Valve has done with TF2 over the years. I've been playing since 2008, I've run servers, I've played both casually and (a little) competitively, made
very close friends through the game, been involved in the mapping community, etc. so suffice it to say I have some very strong TF2 opinions. Yeah, they've totally sent the artstyle to the shitter, and made the game complicated in some very needless ways. (And don't get me started on the shitfest that is the shady, gamble-y way loot works in Valve games, not to mention what's going on with CS:GO.)
But you know what? TF2's still a damn good game, because it's still fundamentally TF2. The core of the game hasn't really changed, and I can still hop in a Dustbowl server and have a good time even if I've played hundreds of hours of Dustbowl before, and there's very few games I could ever say that about. While there will definitely be people who leave TF2 for Overwatch, I think there will be less crossover than some think, especially from a competitive standpoint, because it's really at the high level when the differences between the two games start to become very apparent.