Borg was done at 25. He wouldn't have won any more GS from then on, Borg could barely win a match from 1982 onwards.
He probably would have won at least a couple of Aus Opens if he had been bothered to go there in his peak years. Connors however only played the event twice I believe, winning once, and didn't go after 1975. Mac missed the event 1979-82 and again in 84, his best year. They may well have added to their GS totals also.
Borg was just not able to win the US Open for some reason. In 1976, Connors had his number and was the better player then. Borg was only 20 and still improving. In the 1977 tourney, Borg retired to Stockton. Connors blew Borg away in the 78 final.
79, Tanner blew him away with the big serve under the floodlights to gain revenge for the Wimbledon final. In 80 and 81, Mac was the young man on the rise and improving, Borg was fighting to keep his hold at the top. They were 1-1 in the big finals 1980. By the time of the 81 US final, Mac was better than Borg on the big occasion and on that surface.
It is wonderful that
Vilas could win Connors in Claycourt US Open, but Borg could't (Don't forget Borg was King of the Clay in that time