I think Disney already IS lagging behind Universal and Six Flags in terms of how the FP is used. True, Disney invented the system, but both Universal and Six Flags have taken the concept and improved on it by leaps and bounds. Your suggestion that perhaps Disney could "someday" charge for it, is exactly what those two parks already do, and that's definitely the right way to do it. I rarely even use FP anymore when I visit Disney parks, because compared to how the other parks do it, it's just kind of a pain IMO. You have to go stand in line to get the fastpass to begin with (each time) and then you have to worry about what time to come back and enjoy the attraction. It takes the spontaneity out of the visit, which is something I've always enjoyed about a day at a theme park. I don't like going to a theme park and having to schedule out my day essentially. I get too much of that at work....I'm on vacation!
Universal's system is hands down the best. The idea of tying it to your hotel room key was absolutely brilliant. No extra card you have to carry (just simply show your room key). No worrying about what window of time to come back to a particular attraction, just come back when you feel like it. The Express Pass, which you have to pay for, is also a better idea than FP for similar reasons.
And although I don't visit Six Flags parks nearly as much as the others, their system is much better than FP as well. As Universal does (and as you implied would be an improvement if Disney did this as well) they charge for it. But you get this cool little gadget that lets you "reserve" your place in line on various rides no matter where you are in the park. No standing in an extra line, and you still retain some of that spontaneity. Not nearly as good as Universal's systems, but still much better than Disney's (I never thought I would use the words "Six Flags" and "better than Disney" in the same paragraph, but gotta give due credit for a great concept).
The problem with tying it to your room key is that not everyone has a room key. And the problem with charging for it is that you've
already charged everyone else for it with the price of admission.
My last stint as a Universal AP holder was the last one largely because of how they run their Express Pass system. We made several visits to the park during which we were unable to ride almost anything, because EPs were generally gone by 11AM for most rides, and unlike Disney they seemed to make no effort to limit EP distribution so as to manage the length of the standby lines.
To clarify, they limited the EPs distributed by the machines, but they also sell them throughout the day (gift shops never sold out of EPs while we were there, suggesting that quantities are not limited like they are at the machines, or at Disney). And the ones for sale were good all day for multiple rides on all attractions - which leads to another problem. The way Disney distributes FPs is such that they can predict and control how many FP people are able to access an attraction during any given hour, and therefore are able to predict and control how much standby capacity that attraction has. Universal's system makes it impossible for them to know how many people are going to queue up with EPs for a given attraction during a given time period, and therefore they have no way to predict or control how many standby people they can accommodate, with the predictable result being a ruined experience for anyone who doesn't buy an EP.
It is my belief, and I hope and suspect that it is Disney's as well, that this is just one of the many reasons that Disney's competitors have so much trouble competing with Disney.
That's not to say that I expect, or even want, the FP system to continue as it is permanently. But it is again my hope and expectation that any changes Disney makes will be extremely carefully designed and implemented so as not to take away from the guest experience the way Universal's system does.
The most likely thing to be tried next, I think, is the "queueless" systems recently tested at RnRC and rumored for the new Dumbo, in which you're given a group number, rather than a return time, and asked to wait in a particular area until your group is called. From what I heard of the RnRC test, and the Dumbo plans, there will be things to do to occupy your time while you wait. If it works, I think we can certainly expect some of those "things to do" to eventually include revenue-generating activities such as shopping. I have to think that the minds who came up with the idea of not being able to get off an attraction without passing through a gift shop would likely have thought "if only we could get them through the gift shop on the way in when they have no choice but to take their time, instead of on the way out when they're in a hurry to get to the next thing". In that context, the queueless system seems almost an obvious combination of FP and exiting through the gift shop.