Our covers have averaged around eight years before needing replacement. Each failure was related to edge rope/binding failure rather than any degradation of the cover material. Binding failure on the first couple of covers was likely acellerated by user error (not power washing tracks in spring or monitoring plastic guides where cover enters track for wear).
Part of the original 18 year old cover still functions as a tarp over firewood storage structure. Other than being a bit dirty, the cover material remain functional without benefit of added U/V protection. Other portions of prior covers were used as weed barrier below on-ground decking and those pieces have remaind intact.
U/V protection may help if cover lasts more than a decade, but my experience is limited to a shorter use period. When we had a hot tub, Aerospace 303 was recommended to protect the vinyl cover. The hot tub cover remained fine with annual application of Aerospace 303 for the eight years we had it but I don’t believe the material on the hot tub cover was remotely similar to the heavy material used for the Aquamatic cover.
Joe M in WV