I would not say that Overwatch improved on TF2’s design. Rather I would say Overwatch summarized and copied some of TF2’s design then went and did their own thing.
Ultimately, a large part of the games success came from the Blizzard logo and advertisement investment. The game we started with was… lacking in content. Even now 2 years later it still feels lacking in content, but this is largely because a lot of money and time was invested in temporary content that is held back for the purpose of creating time windows to squeeze money out of people.
Personally I think why this game type became a staple was because people wanted to cash in on the MOBA craze at the time, but there is only so many times you can reskin a moba. This is why some opted to abandon the isometric perspective and take it to third or first person. Which is why they are still viable and around.
There was one competitor to LoL and DotA2… cannot remember what it was called though. It kinda got crushed out of the market by the 2 juggernauts fighting it out over the years.