There is no lack of games. There is only a lack of knowledge on what they have in the pipeline for the coming years. Quantum Break, one of my favorite games ever, came out on Xbox One just last year. Even on the Playstation side, which is seeing a nice set of releases right now, a lot of what we know about is a ways off or doesn't even have a release date yet.
Shenmue
Kojima Title
God of War
Last of Us 2
Days Gone
and that's just to name a few. Horizon Zero Dawn's release doesn't really change this.
Sure you have titles like Persona 5, which will probably be one of the top 3 games this gen, probably #1 for an RPG fan like myself. but special releases like these aren't an every day, or even every year thing. Based on what has already released already, Microsoft has had a nice lineup of diverse games. The problem is, with Scalebound's cancellation, we lost something big people were looking forward to, and it leaves you wondering without knowledge of much else out there in the pipeline for xbox just what Microsoft has. I'm sure they'll more than answer that question this E3. I'm not really worried about that. Sea of Thieves is also a nice looking new IP releasing this year. And while I wasn't a fan of the previous crackdown games, maybe this one will manage to surprise me. Microsoft has games, they just need to announce more. We can't honestly expect Microsoft to have a new major IP every single year. It's not realistic and certainly doesn't occur that way for any other platform. I see Quantum Break as a pretty major release from them and an excellent game, and that was just last year. I think people overlook that fact.