I thought the idea of the leasing of the Shangri-La was to replace HMS Hermes, which had after trials been deemed unsuitable for the future operation of the F-4 due to it being too small, as only Ark and Eagle were suitable.
So, Hermes would would have got the chop instead, and we'd still have had the 3 proposed carriers, although I think it would have been more than 1000 extra reqd to crew it, as Hermes was 2100 crew and USS Shangri-La was almost 3500.
Of course, as mentioned in the OP, only Ark ever carried the F-4 with the better condition Eagle getting the chop, and Hermes continuing on into the post cat n trap era.