owlflatlinesaying that overwatch will kill tf2 because it has classes and it's cartoony is like saying heroes of the storm will kill dota because it's a moba
Overwatch could definitely kill tf2 if all the good players leave for it (which seems to be happening in EU I guess)
That's not killing tf2 though. That's killing the current competitive format of tf2, about which only a tiny percentage of the tf2 players care about. Dont worry, those 24/7 dustbowl, 2fort and instant respawn orange servers aint going anywhere.