they can if they want too... you may feel it will be a disaster, and I agree it might be, but if they want to do that, they most certainly can. They can sell a limited number to offsite guests.... Maybe they would do a tier system for their onsite guests, value resort gets so many free, then after that, they get a discount, moderate the same thing but more free than the value..and the deluxe and home-away-from-home get all free... You cannot say never just because you feel it will be a disaster... again, I agree with you that it can possibly be (if not done correctly), but since when has anything stopped Disney from doing what they wanted to. There are ways they can do it...
Either way you look at it, they are free now, and with all that are using them, it has not turned into a complete mess. If there was a fee, you bet your bibibidy boppity boop there won't be as many using them. All that are staying onsite now are using them, aren't they?
by the way, if you noticed, I did not say "unlimited" attractions.. I said "so many" and "all". Universal's fee based Express Pass doesn't give you unlimited. Onsite does (or at least it did last time I stayed onsite).