When F&M was just ending and there was the limited re-opening in Snowy, all the carparks were closed save Llanberis. The Sherpa bus did loops coming and going every half hour, with an unlimited ticket costing the princely sum of one quid. It ran from 8am till late in the evening, and frankly, it was very successful.
This is the sort of service you could expect to see if Green Key was implemented, though the larger car parks such as PyP would still be open albeit more expensive. Some of the more random laybys would be closed but those for Dinas Mot, Cromlech etc maintained.
After driving around a bit on Good Friday and not finding a SINGLE space in Llanberis Pass, PyP or PyG at about 11am, I honestly can't see how you could argue against GK for peak periods, as the current arrangements just don't work.