Murray didn't have the tools to threaten Fed's game in 2015. And yes, Fed's game was vulnerable in 2015. The serve was absolutely stellar but other areas of his game like groundstrokes and especially returning were average to weak. Murray was just so passive in 2015 that he was never winning that match though.
Peak Nadal, of course, is a bit of a better player than 2015 Murray and would have the right tools to take Fed down. The serve would be very tricky, but in a prolonged match I doubt Fed maintains that raw serving strength the whole way as it was a bit of a lightning-in-a-bottle performance. The shot very much returned to earth in the final against Djokovic.