Over watch is a go-go-go 1-2-3 simple arena shooter, I would agree nobody will be able to touch COD4's multiplayer the absolute prime of online multiplayer gaming, overwatch didn't intend to be #1 but it can hang.
Dang I never got to open my 1000+ loot boxes that I collected since the beginning. Maybe if they bring back the 6v6 as an Arcade mode I will be tempted to go see everything I had gotten.
The issue with modern life service Games. They will change and either adapt and follow or go home.
I know times when different leagues played different mods for games (e.g. ProMod for CoD4 in the ESL while Clanbase used a different one, IIRC) because of different opinions in how it should be balanced and be played. You would find servers running different settings, mods, etc. To be fair, they had a server browser with custom settings in OW1. I don't know if you could earn XP on these though.
But that way you fragment your playerbase too much which could hurt in-game monetisations, the only thing they really care about.