Nope, started with Reach and Judgement. We actually just had a thread about Gears specifically, too, where this was discussed.
And the "small userbase" excuse doesn't hold up when Gears 1 and Halo 3 both had relatively small bases to work with on 360, yet both ended up doing better than Gears 4 and Halo 5 despite the Xbone having higher sales than the 360 launch-aligned.
Check this:
As of now, it's a trend. That isn't to say the series can't rebound or even do better than they ever have, but that remains to be seen.