I kind of agree with the innovation angle since Siege, a 7 year old game, is still the one with the most advanced gamplay systems in 2021 (granular destruction, sound that follows the shortest path and the ability to shoot during most animations)
On the other side, multiplayer games have extremely long lives now, which might be part of the problem since you can't change much the games without upseting the player base which becomes huge when games are getting supported for at least 10 years when really successfull.
Edit:
The time to kill is too damn low. If people just want to spray and pray with low skill weapons and watch people drop fast, then there isn't much room for gameplay innovation that encourages people to learn new things. Unless you want to concentrate on "non-combat" innovations like scavenger hunts and ten dimensional inventory tetris.
Have you ever played Rainbow Six Siege? extremely low ttk but is full of gameplay systems that you don't see in games that seem similar on the surface.
As an example, maps feel huge compared to most other games not because of sheer size but complexity since you have to not only understand the layout to have good awareness of what is going on, but also what is destructable, what not, and how has it been modified by the teams in the round.