I disagree with you saying Overwatch doesn't have a steep learning curve, if anything it's more steep than Quake.

The difference is that Overwatch is extremely easy to start playing as a new player and still having fun. You don't need to know the advanced stuff to still win and have fun. However the game gets super in depth both on your independent skill and ability to coordinate with your team.

The teamwork skill is something Quake doesn't have. Also being new in Quake, you get absolutely destroyed and until you learn some basic movement abilities and learn the maps, you will keep getting destroyed.

I agree with Fortnite though, non-steep learning curve and shallow gameplay overall.