A lot of good points are being made here - pretty cool that they featured this post, appreciate it! The general consensus appears to be that Halo 6 should be released in Q4 of 2018... that said, I'd love to see that release pushed back to 2019, perhaps to coincide with the release of Project Scorpio (which, I believe, is still TBA).
I'd argue that Halo 5 still has pretty strong legs and a lot left in the tank, should 343 choose to continue to support it. Here's why...
Genre of game: Obviously, Halo 5 is an online shooter. Which means that, relative to 90% of other games on the market (indie, action-adventure, perhaps racing, sports, platformers, etc.) it has a LOT more staying power due to the sheer amount of content provided. For those of us who already love the game and have invested so much time into it already, we want to see it through to completion at this point as a result. So even if, say, Telltale pulls me away for a 10 hour stretch, Halo 5 will bring be back for the long haul so long as the player base remains.
The "Niche" of Halo: Think about other shooters currently available on the market. Can you name one that even compares to Halo for what type of shooter you enjoy? I can't, and outside of Destiny, I've played them all. Halo offers an epic, sci-fi story and lore I've always compared to that of Star Wars in the movie world - Halo as a franchise is akin to that in video games. It offers short, competitive MP matches, larger scale, vehicle-oriented battles, Warzone, and some fun, messing-around gametypes as well (Grifball, Action Sack, Infection, etc.). Basically, it has it all in my book, and no other shooter has matched it yet for me. COD = fun campaign typically, but too much "run and gun, get the first two bullets in a guy and they're dead gameplay." Gears = 3rd person shooter, which doesn't do it for me. Battlefield = epic campaign from the sound of things, but large-scale combat only, nothing smaller in scale and competitive. I loved Titanfall, and Titanfall 2, but the release window failed that game completely, who wants to invest time into a game when the player base is going away already? And Overwatch, though I hear it has gotten better with arcade mode, I just couldn't get into at all outside of a few hours here and there (think Rocket League). So it's very different.
The Halo / Xbox Community: This is a big one. Name one other game with a community nearly as good as Halo's? I don't think one exists. With the Pro teams out there, the casuals, the try-hards, the Company members, the Forgers, the guys on Waypoint, the custom-game builders, the writers, etc... no other game that I'm aware of is even close to Halo in terms of community support and fan-base. Even people who have moved on to other titles through the years want to come back to Halo from time to time - I have several friends who have moved on but occasionally check in and play a few games because "well, it's Halo!" And there are very good odds that if you bought an Xbox One, you bought it for Halo, or maybe Forza, but everything else would have told you to go PS4 at the time, so it's not like that has changed.
Last, but not least: Though the player-base has obviously diminished, do you have trouble getting into Halo 5 matches basically 2 years after its release? I don't. Whether it's over lunch, or 10 at night, it typically takes 30 seconds or so to find a match, even if it is the #20 most played game on XBL (as someone posted above). Regardless of the playlist I try to join. I can definitely see that sticking with such devoted fans, with Destiny 2 really being the only thing I fear as a nail in the coffin to Halo continuing to thrive. Many people said the release of Halo Wars 2 would kill Halo 5 - but it hasn't. In fact, most of the guys I hear going to that game are coming back to 5 now. Would 343 really want to release Halo 6 so soon after the release of Wars 2? Maybe... but I really hope not.
Sorry, done with my two cents... the bottom line is, I hope Halo 5 carries us into 2019, and that 343 takes there time making Halo 6 the best it possibly can be. I really think, with a little more continued support, and assuming Destiny 2 doesn't blow people away, that 5 has the staying power to last us until then.