The last Halo that managed to stay in top 5/top 10/top 20 for its lifetime was Halo Reach. It held on in the top 5 despite having sweeping gameplay changes that frustrated fans and going up against an incredibly strong Battlefield, COD, and Gears at the time. Prior to that Halo 3 managed to survive 3 years going back and forth between #1 and #2 against three separate COD titles-COD 4, WaW, and the juggernaut that was MW 2.
While the console shooter market has changed and there is more competition it hasn't changed that significantly. Where the old market was made up of COD, Battlefield, Halo, and Gears, the new market is COD, Fortnite, Apex, Rainbow Six, Apex, Destiny, and Overwatch. There are more shooters out there but they all comfortably sit in their own space with very different gameplay mechanics.
Contrast that with Halo. Halo changed with Reach, 4, and 5 all shedding or modifying core gameplay principals. The game adopted mechanics that made it closer to other games on the market. As a result fans of the core Halo gameplay were displaced and the population dropped.
Halo 5 is a decent FPS. It is mechanically pretty sound and it has a satisfying gameplay loop. I think it would have a decent albeit niche audience if it were F2P and separate from the Halo IP. When people buy a multiplayer sequel they come in with expectations. People expect COD, Battlefield, Street Fighter, Counter Strike, and Halo to look, feel, and play a certain way. It is up to the developers to meet that audience where they are. Halo 4 and 5 both failed to do that. That doesn't make Halo 5 a bad game but it does mean that it fails to excel as a Halo game.
There is a rabid fanbase out there that wants that classic style Halo. It doesn't mean that there isn't a market for Halo 5 MP it just shows that there are two separate markets. Rebrand Halo 5 MP as "Guardians" or something and put it out there-I think it would do alright. That said, I believe that the market for classic Halo is larger. If you look at the top XBL charts today Halo MCC, a game with old clunky netcode, is ranked comfortably above Halo 5. A refined Halo 1/2/3 would do well today with modernized features: centered crosshair, 128 tick servers, high FOV, modern netcode, etc. Some counter and say that a game like that is too clunky and punishing for todays' audience. I disagree-look at the success of CSGO, Valorant, Overwatch, and Rainbow Six. All of those games can be incredibly technical and punishing for newcomers yet they remain as some of the most played shooters on the market. People are hungry for those competitive shooters.
Where I think Halo traditionally did well was with it's sandbox. Halo is a competitive shooter at its core but the sandbox allowed for people to dress it up into a more casually appealing package. Big Team Battle with Tanks and Hogs, Zombies, SWAT, Snipers, King of the Hill, and endless crazy custom games built with Forge were all layered on top of the core competitive gameplay and Halo saw tremendous success as a result. When the gameplay changed it lost this appeal. That core competitive experience was lost. As a result the casual experience that was built on that foundation was lost as well.
Microsoft should give classic Halo a chance. They changed a model that was working well and have been met with a lackluster response. They might be surprised with the response to a return to form.