This I don't agree with. He would still be the most successful of that lot. Juan's peak was for like 4 months and he was still beatable at the time. Look up his H2H against Murray.
Although Andy Murray is the most consistent player of that group, the problem with him lies in the fact that his absolute peak isn't as good as the absolute peak of those others. His game revolves around neutralizing his opponent's strengths and relies on them making mistakes. Problem with that is if such opponents are firing on all cylinders and aren't making much mistakes, Andy Murray simply can't beat them! Simply put, Andy Murray lacks the 'firepower' and the match winning abilities to proactively win his matches, irrespective of circumstances.
So it'd take only one of those players with such 'firepower' to be at their absolute peak, in order to diminish any chances of Andy Murray winning a grand slam. For Andy Murray to win, he'd have to rely on every one of those players to not be at their peak, which is highly unlikely.
To further support my argument, a handicapped, one handed version of Juan Martin Del Potro who is arguably at his absolute worse forced Andy Murray to go life and death with him in the last 3 matches they played. Del Potro lost in the Olympics 2016 final in a very close match then got his revenge in the Davis cup match. All very close matches. If Andy Murray has to win by such small margins and has to go life and death against a handicapped version of Del Potro, what do you think a peak Del Potro would do?
If Andy Murray was so much better, then he should've won far more easily and decisively against a handicapped version of Del Potro. That wasn't the case however!
As far as Stanislas Wawrinka is concerned, I'd favor a peak version of Wawrinka to beat any version of Andy Murray outside Wimbledon.