I think if you look at the Halo franchise they had Halo 4 which people didn't like the multiplayer of and played other games instead which may have been damaging, but they also had MCC which was broken and shouldn't have released in that state and that was definitely damaging.
Compare it to Gears, Judgment is a bit like ODST, it was $60, ODST lacked its own multiplayer mode which Judgment had and people didn't like and it lacked Horde mode but the campaign (imo) was still good, it was a lesser selling title but so was ODST and that didn't seem to cause any significant issues to Reach which launched a year later.
Gears has had 2.5~ years between Judgment in March 2013 and Ultimate Edition in late August 2015; Ultimate Edition looked great, faithful to the original, it worked and includes Gears 1-J in their Xbox 360 form for $40... I think Gears of War 4 will have a good release given its had the one thing Halo hasn't had and thats a nice rest. Theres been something Halo related released at retail at the end every year since 2007 with the exception of 2008 (but two titles in 2009) and 2013 (I don't think Spartan Assault/Strike are particularly relevant to be included).
Now, Gears 4 may well be down on Gears 3 opening, in fact i'd say its very likely but by "Or Gears could drop as well" I assume we're talking on a similar level to Halo 5 rather than just a not quite selling a much. The Halo figures in NPD show (I believe) Halo was selling about the same amount of copies at launch in NPDs across 3, Reach and 4 but each time the game released earlier in the month (25th - 3, 14th - Reach, 6th - 4) which surely is somewhat of a decline just not necessarily such a glaring one (as 5 is anyway).
If Gears of War 4 hits all the right notes I think that will come across well to consumers, the franchise has had a decent rest and without the glaring and more minor issues that Halo has had...
I think there are multiple reasons why Halo is down, 4/MCC are important, franchise fatigue(?), Xbox One user base doesn't seem to be buying much in the way of exclusives to be honest (an issue in itself), perhaps other multiplayer games of a similar type have just eclipsed it... I don't think Gears has the exact same competition.
We'll have to see what happens, but Gears is a better franchise than Halo anyway so if it becomes top dog then praise be Rod and i'd love that. I think Microsoft focusing on a fairly narrow range of titles is becoming a problem for them, though.
