I think it's possible for Microsoft to release Halo 6 on the PC as a way to test the market, but I do see them as also being hesitant to do so as well given the... reception of Halo 2 Vista. Of course, it's also possible that if they do release Halo 6 on the PC, it would be exclusive to computers with the Windows operating system, as a way to make sure that all of the money goes back to them, just like they did with Halo Wars 2. I think, however, the question lies in whether or not Microsoft sees releasing a Halo FPS on the PC as viable in comparison to simply releasing the RPG games (HW and HW2) and top down shooters like Spartan Strike and Spartan Assault. I also believe that we need to look at how successful Halo Wars 2 has been so far on the PC, as it was released for both the X-Box One and the PC, and therefore its success and/or failure could be an indicator of whether or not Microsoft will release Halo 6 on the PC at all.
This, of course, brings up another topic, which is whether or not we will see the release of a Halo 6 Custom Edition to coincide with a PC version of Halo 6. I can tell you right now that Halo CE has a large and active community, while Halo 2 Vista does not, mainly due to the lack of mod-friendly features and tools in H2 Vista's "custom edition" features. When Gearbox released Halo Custom Edition, they gave people the Halo Editing Kit alongside it, giving fans access to all of the tools used by Bungie to create the first game. Halo 2 Vista, in contrast, had a drastically stripped-down editing kit, and nearly all of the tags and assets found in the single player campaign were locked and unusable, which meant that the game would go on to be ignored as people stayed with Halo CE. Also, H2 Vista was not ported very well, and it was buggy and it had numerous problems with limits on installations and crashes and... ugh.
So, in my own opinion, I believe this is a case of waiting and watching. I hope that Halo 6 is released on PC, but we also have to think of what the system requirements will be for it as well. I suppose only time will tell.