The whole genre of linear single player FPSes is sort of dead. Lots of mixed genre games that tie in RPG elements with big open worlds and character customization though that are doing quite well and could benefit from better AI.
Even with a friend though we can't reliably beat the top level AI in Ghost Recon Wildlands so it can't be that bad.
I personally like the interactive movie feeling, the scripted events and the scenes. I did never enjoy CoD in multiplayer which I find too hectic. The battlefield series with a more slow-paced and strategic gameplay suited me better in multiplayer. However I generally don't enjoy competing in MP that much anymore - too little time and interest to train enough to be competitive with the younger players.
Linear single player FPS is definitely not dead, although multiplayer is the current focus. As a quick example, DOOM (2016) has both linear single player and multiplayer sides, but the single player side was much better received.
I suspect that there's an element of self-selection going on here; there are lots of people who play the single player, but an onliner can't see them because when that onliner goes online it's 100% people who go online that one sees there.
There could be ten times that many playing it for the single player and they'd remain completely invisible.
I did, when I actually bothered to play CoD (the first Modern Warfare was probably the last one I enjoyed) and couldn't stand the multiplayer.
When I had been used to games like Quake, Unreal and Battlefield the multiplayer in CoD always seemed so half-baked.
The genre seems well and thriving.