I think an issue with Gears of War is how tough the multiplayer is. It's great for competitive sessions and e-sports and all that but the learning curve is insane, and the lack of options for new players to pick it up and learn the mechanics (even just aiming and running and going into cover, all three basic things that I see new players struggle with) is really cutting down on the number of customers.
I've been playing Gears since the first one, and while I'm not great at the multiplayer I can pick it up and generally go positive (I think my K/D right now is like a 1.1 or 1.2?). I'm not great but I still have fun. That said, I still get frustrated because there really aren't any options for weaker players. My options are to go into social quickplay, which will occasionally be match me with average players but almost always there are 1-2 people who just go 15-1 or 12-2 or something similar that ruin it for everyone. I can deal with it because I know those players aren't really common and I can work everyone else and hope things go well and the good players screw up.
Otherwise I go into core modes which is the same situation as above but worse, or the competitive which is even harder. Yes I know the 5 qualifying matches bring you a rank that is supposed to help you out but in the only two that I have fully completed those 5 (Dodgeball and Warzone) I still have the same issues. One or two players just destroy everyone. It ruins the fun. And I'm what I would call an average player, or hope to be called that. I can't imagine how dejecting and demoralizing it is to be someone who would be considered bad or below average going against these guys.
Meanwhile, I try to get friends to buy it - a crew that generally all gets the same games together to play together - and none of them want to. "It's too hard" is a common saying. Two of them agreed and said that buying Gears 4 would only be for campaign because the multiplayer is full of people who "only play Gears. They're all way too good at that game." None of them knew Horde mode is a thing. When I explained it, they all thought it could be cool but they want to go against other human players and with the brutal learning curve and lack of introductory learning they simply won't. This group buys damn near every multiplayer game together and they all skipped Gears. Looking at the sales numbers I can't help but think it's a common thought process.
Coalition (and yes, I imagine they're going to keep making only Gears games because they changed their name to reflect that - Coalition, Coalition of Ordered Governments [COG], much like 343 Industries and Halo) need to do something to open up the games to the general public. E-Sports is obviously growing but when everything is done to appease that crowd they lose the much larger general group that just want to have fun.
Looking strictly at campaign and horde: Campaign was really great overall, but the opening was really, really bad. People who got past that initial Act probably had a lot of fun (aside from a lackluster finishing endgame boss). Horde is fun but I can't get into a full game. I've tried three or four times and people always quit before it starts and then by wave 5 you're down to 2-3 people. Matchmaking is incredibly bad for Horde.