At first I liked the idea of a cap. But upon thinking about it further I don't like the idea. I would feel obligated to hit the cap every week even as a semi-casual raider. If they made the cap really high like all WQ's and 20~ MoS/week that would probably be okay. That cap is so large I know I wouldn't ever hit it. Top end players would still have to work hard to max out there weapon, but they wouldn't feel obligated to run 50+ MoS/per day.
I like how the ring from WoD naturally nerfed HFC, I want the artifact to do the same thing, but with how AP is gained I am not sure how it could be achieved.
In 7.2 it is my understanding that ak research is increasing as well as new traits unlocking and ToS is coming out. WF guilds will not be able to max weapons before progression which means that they will aim for maybe -40ish traits (I have no idea here) whereas the cap is much higher than that. I don't think this is really going to be an issue moving into 7.2 for high end guilds.
However atm can guilds collect tons of AP and throw it into a level 53 weapon and have those points transfer to 7.2? They could collect lots of AP, store it in the bank, and right before 7.2 hits reset their weapon and throw all the points into it so that they could get 6 or 7 traits on opening day instead of 3-4. Blizzard needs to make sure this isn't possible to hard core people don't feel obligated. ToS should be tuned around ~40ish 7.2 traits.