AR will only be useful for gaming when AR headsets become dual AR/VR devices. Other than that AR gaming will, for the most part, be gimmicky rubbish which gets old fast, "ooooo look at this computer generated character jump around my room...oooooo..", much like the mobile phone games of today (yes, they tick the box for many people.. for some reason...). AR really shines in real world application, navigation, media consumption etc. It could also be nice to bring the likes of pinball, tabletop games and arcade cabs into your home virtually. Traditional "Computer Gaming" not so much. Why would I want elements of a game projected onto real world objects when I could remove the real world entirely and be immersed completely in computer generated ones? Of course that is preference, but I'd choose total immersion every time.
I've spent time with a Hololens and one thing I'll say, if you think VR has a long way to come, AR devices are way, way, waaaaay behind at the moment. The concept is there, the devices are not. People say that the form factor of VR needs to be a pair of glasses, I say it doesn't it just needs to be functional / comfortable and it is right now, AR however 100% needs to be the form factor of glasses or even contact lenses to be remotely viable - people are not going to be going about their daily lives with a hololens type contraption strapped to their heads.
Will VR replace the monitor when it comes to gaming? In time I believe it will. By the time AR matures enough to be viable playing games on flat screens will be a thing of the past. We'll all look back at flat screen gaming like we do at the 8-bit generation. It will seem comical that people wanted experience games through a small window rather than being in them.