Leaks suggest that they're going to both get spinoffs this gen. I have a feeling that Microsoft is saving Halo 6 and Gears 5 for next gen at this point. Microsoft so far has only released Halo and Gears games on the Holiday, and so far has been unwilling to release both Halo and Gears on the same year. Taking that info into account, If the Halo spinoff releases Holiday 2018 and the Gears spinoff Holiday 2019, then the first possible window for Halo 6 to release is Holiday 2020, which is when the next Xbox will most likely release, while the first possible window for Gears 5 is Holiday 2021. Of course MS could break the cycle and release outside of the Holiday, or release both a Halo and a Gears game on the same year. But I think at this point MS does want to save up some big games for next gen so that the next Xbox has a better chance of success than Xbox One did.
We should also take into account the fact that The Coalition is very busy. If the various leaks and rumors are true, they have teams working on 3 other games right now besides Gears 5. Supposedly The Coalition and an unknown studio are co-developing a new Perfect Dark. The Coalition and another studio are co-developing a game based on the Dark Tide short story. And The Coalition has a team assisting Splash Damage with the Gears spinoff I believe. With them split between so many different games, development on Gears 5 will most likely be slow.
It's makes sense to save these games for the start of next gen where they can make a bigger impact, specially for The Coalition if they're really working on so many projects. But the problem is that it could give the impression they're abandoning ship, which never resonates well with console owners, specially the ones who aren't early adopters.
"lupus in fabula, venit enim ad me."
Translation: I will always hate Fox for canceling The Exorcist.