You're right that the gap is widening between Nadal and Murray, but I also think it is widening between Djokovic and Murray.
With Nadal and Djokovic totally a lock for every grandslam semifinal at the very least, there is no way either of them are letting Murray through to the final at any slam, therefore I think Murray will never reach another slam final, let alone win a slam.
That has no logic. Fine, Nadal won at US Open in four so they are 1-1 there, level at Aus Open 1-1 too. Murray beat Nadal from a set down in Tokyo final, Rafa winning FOUR points in final set. So, certainly on hard court it's not too bad. Murray took a set from Nadal on Grass for first time last year too, hell, he even had chances at French Open.
Last year Murray loses to Nole in three in Aus, this year in 5. And has a losing 7-4 Head to Head. It's hardly widening, regardless of you're obvious Murray distaste.