Eh, popularity is one stellar game away. Hit the right notes and gamers change their tune pretty quick. It's like wild brush fire, when it catches off it goes. Given launching with Scarlett, xCloud, on PC and more devices we're going to see quite a resurgence of Halo. Similar to Fortnite or Apex I feel, whether they capture or manage the population, content sustain and/or cross play so it continually grows is another story we have to wait and see to read.
There has been a shift away from arena shooters, there is more competition in FPS console games since mid life H3 and Halo is yet to really bring its PvE game to town e.g. Destiny. They have to expect such a decline with their MCC fiasco and poor Xbox One launch that took years to recover (they've done a stellar job in recent years course correcting), a divisive hardcore focus on esports with H5 and multiplayer audiences have shifted to PvE based games as well as BR style games where other elements exist besides simply straight up killing your enemy on a small symmetrical map.
Players enjoy looting, large map movements, larger weapon sandboxes and more that isn't inline with arena style games. 343 also ignored the core 4v4 and 8v8 aspects of light vehicles slayer/objective modes for two games (H4 & H5) which placed a very core audience segment for Halo out of the limelight for more than 5 years by the time Infinite launches.
Infinite still has all the potential in the world but has much promise to deliver on. Halo 5 progressed for some 1.5 - 2 years post launch and finally got the regional networking stable and reliable to play very well in game. It took years for MCC to get there as well, that doesn't go unappreciated but should never happen in the first place. So much of that was driven by the Xbox platform as well I feel at the time, constant change, dashboards, API, Azure and leadership misdirection. There's plenty to be happy about with Halo and 343 in recent years, I'm really quite excited for Infinite, we'll see what comes of that and how the global population responds to it.
Hopefully Infinite captures that world wide audience off the back of included with consoles, game pass inclusion, PC+devices ease of access. There is parallels between modern BR games getting so popular and some additional offerings from Xbox/MS that point to all the right moves so far. The game has to deliver at launch though to create that buzz, attract and retain players while not going down that slow burn course and content correcting pipeline H4, MCC and H5 did.