Well there was Halo, Gears of War, Battlefield etc. Competition may be greater now but these games have a far greater reach globally and are more accessible plus the likes of Warzone can be played for free. Overwatch, PUBG, Halo are all dead in the water now so really COD should be back up there yet it's MP player counts are less than Goose Goose Duck. It's not like people aren't buying COD now - they still mostly achieve the same figures they did back then but when the population nosedives after a month and never recovers then it tells a big story no matter how much you seek answers elsewhere as a factor. Sony are fighting MS because the acquisition could severely damage Sony's system sales going forward especially if Microsoft are to lock all the games to their consoles now and til the end of time which they will.
COD is also never consistently at the top of Steam - it's barely cracked the top 5 once in 7 years and currently sits in 11th based on current but even the peak wont see it get into 5th.
So sales, engagement and popularity isn't the issue but a game losing this much players so soon especially when it was two years to run is a worry. A lack of content and ongoing strategy plus the game releasing in an unrefined state certainly are the biggest factors in these huge declines. The market is there, they have their captive audience yet they persistently blow it.