Halo 5's Forge level editor will be released on PC for free, but it doesn't look like Halo 5 will follow in its footsteps. However, it doesn't look like Halo 5 itself is going to follow suit. Halo fans need not worry about the future of the franchise on PC, as Microsoft has already revealed that Halo 6 will be released on Windows 10 PC.
Xbox head Phil Spencer reassured PC Gamer that future first-party Microsoft games will be released on PC. When asked if there would be any ideological reason as to why the next Halo mightn't be released on PC, Spencer replied with a firm, "Not at all."
He was also grilled as to why Halo 5 wasn't released on PC, and he replied that Microsoft and 343 Industries are looking to the future. "Honestly the answer with Halo 5 is, I can go take last year’s game, rework it to go on PC, or I can have 343 look forward in what they’re going to go do," he said. Spencer also drew a parallel between the Halo and Forza series. Halo Forge is coming to PC, as Forza Apex recently did; Spencer added that in the future, Microsoft will "bring out Forza games to PC complete." There's a small jump in logic, but you could say that future Halo titles will be given a similar treatment.
Add to this the fact that every first-party Microsoft game revealed at E3, including Gears of War 4, Forza Horizon 3, and Scalebound, will be released on PC as well as Xbox One.