[croon]

Or it could be that Overwatch is 9 years newer.

[scrollaway]

You mean, that TF2 has had a 9 years headstart in improving and polishing their game's design, add more content etc?

[croon]

It's hard to throw out core principles or do massive overhauls without starting over. At that point it's no longer a patch. They had many years to consider what made a game good, and what to improve on, and modernize concepts.