All of my M+ group is between 48-51 right now. The system is serving us well because every 2 weeks or so we get another trait, but we don't bother grinding it out because we don't need to. We will all be finished with our weapons well before 7.2 hits. I think you're a bit out of touch with casuals though. Casual doesn't necessarily mean "just started playing", and many casuals are AK25. At that level, doing emissaries for a week will get you to 35 in a weapon, I'm sure most people playing casually since launch are in the 40s.
But the main point is this: the current system of AP to trait 54 is awful. But we aren't talking about the current terrible system to 54, we are talking about the 7.2 version (more on that below).
See, this is the main point. The current system is awful, you are forced to do M+ which is why they are changing it. In the 7.2 system, if you were to run 100 M+ dungeons between raid nights, you would gain a 0.05% DPS increase (~2 traits) with the new paragon system at best. Do you feel obligated to run 100 M+ for 0.05% DPS? You certainly shouldn't. AK scales so fast specifically to DISCOURAGE people from stupid grinds to finish right away, you will get as much AP from 100 M+ maw runs as you would get from your regular normal mode raid with 2 more ranks of AK. If you feel forced to run M+ dungeons the way the system is set up on PTR currently, I don't know what to tell you, you are probably over-valuing the infinite trait. It is a functional cap, no different from an actual cap except that it doesn't make running random stuff with friends completely worthless, only mostly worthless.
That's basically how it works on 7.2 PTR now. If you try to grind traits past a certain point, it will take HUNDREDS of M+ runs to get one trait, for a 0.05% DPS gain. But if you wait a week and collect some AK, with how fast and high AK scales now, it will take 10 runs to get that same amount of AP. It is a functional cap, actually pretty well implemented. The main concern I have is how they work a catch up mechanism, since it punishes people who forget to research AK or who just started playing unless they address it.