I think that Connors should have won 1977 Wimbledon against Borg as others have argued here, but Borg should have won the 1978 US Open.
He played the final against Connors with a painful thumb injury and had taken a cortisone injection just before. That final was effectively a walkover, and I doubt Borg would have even played in it had it been at a smaller tournament.
He had clearly overtaken Connors in the pecking order of men's tennis by then, and had no fear of him any more. I think a healthy Borg would have a great chance of winning that elusive US Open title and the third leg of the grand slam in 1978.
Plus around that time his close friend Ronnie Peterson died from a pulmonary embolism following the 1978 Italian Formula One Grand Prix at Monza, so that was a tough time for him.