The concept of subscription fees, membership fees or whatever you want to call it, has been canvassed before. In fact, it was actually tried with a forum that was setup specially for it. It was a massive failure and I'm fairly sure that PPRuNe management won't be in a hurry to try the idea again anytime soon - unless their hand is forced by other events, of course.
I'm sure that the Admins will be able to tell you more about that attempt to accede to their members wishes. And, yes, the push had come from Prooners, well, some of them anyway.
As for the concept of proving our entitlement to a particular forum by, for example, quoting a licence number, WON'T ALWAYS WORK. The fact is that there are folks on here from many other, perhaps less advantaged parts of the world, where the Aviation Regulator doesn't provide any licensing information online. Yes, I'm in one of those places, but I'm far from being the only one in that situation.
Danny and Co have a very difficult time running this place and I, for one, have no problem with abiding by their rules. And, if they end up having to impose a membership fee of some sort, I will be happy to pay it.