WDW does not restrict who can ride their transportation systems and I'm glad they don't. They want people to come and see all that Disney has to offer and this is more easily done by hopping around on their transportation systems.
I think the number of people that will make the effort to get to a location to ride the Skyliner will be minimal. Some yes, but not enough to affect my trip. Pop Century alone has 2,880 rooms, Art of Animation has 1,120 and CB has 2,112 rooms. Just those 3 resorts alone have close to 20,000 people staying in them. Do we really think 'extras' riding for fun will even dent those numbers? Personally, I don't.
I think you would be very surprised as to how many people ride things without paying for them, but I have no data to back this up. Where else can I ride a plane, bus, boat, train, gondola (I can think of only 2 free gondolas) just to "check it out" without paying for it? The issue I have is that I am paying for this, so why shouldn't they?
Restricting access to the Skyliner to only park /resort guests will never happen as long as it's considered a mode of transportation. If you want this, you have to do the same thing to buses, boats, monorails and the ferry at the TTC. Now with security being moved to the TTC, that changes the ferry a bit but because they are not scanning MB's at security, a non park guest could still ride the ferry.
It's a lousy idea from a business standpoint. It's a transportation system, not an attraction. However it is a transportation system that could be an attraction. So, it either stays as is, meaning open to all at no additional cost, or Disney sells tickets to it and all other forms of transportation.
Be careful what you wish for as your wish might just come true. Remember in Tokyo, the monorail requires it's own ticket. Charging for transportation could be implemented at WDW, which would be a huge cash cow. Resort cost won't go down either. That transportation charge to the resorts will just be allocated to another line item.
It is already a line item that I pay for as a Riviera DVC owner. Anyone who is a hotel guest along the gondolas or park guest is already paying for it as well as a line item. Guests at the Tokyo Disney resorts get a monorail pass. Source is the official Tokyo Disney site:
Convenient Monorail and Shuttle Bus Service for Free
A complimentary pass for the
Disney Resort Line monorail, which links the major areas of the Resort, is available for Guests staying at
Tokyo DisneySea Hotel MiraCosta or
Tokyo Disneyland Hotel. Guests who wish to use the monorail can request a pass at their Hotel or at the Hotel Service Counter on the station level of the Tokyo Disney Resort Welcome Center. Complimentary shuttle bus service aboard the
Disney Resort Cruiser is provided between the
Disney Ambassador Hotel and the Disney Parks.
https://www.tokyodisneyresort.jp/en/hotel/dh/privilege
Also, I believe you can walk instead of using the monorail in Tokyo, so that comparison will not work here as you cannot walk to AoA, Pop, Riviera, or CBR from the parks. If I could walk versus anything else, I am going to walk unless the weather is horrific.
Bottom line, I don't think people should be paying for the gondolas while others ride a mode of transportation for free. Let them pay like we do. That'll cut down on the crowds IMO. If you are staying at one of the resorts which the gondolas service or going to the parks, then you have paid for the right to ride the gondolas. If you aren't, then you need to pay for a pass. Whether it's a minimal number of people or a large number of people riding for free shouldn't matter. Everyone who rides should pay for it rather than only charging hotel and park guests.