Name another top flight singles player other than JMac who consistently won titles in doubles...never in the open era (and yes I know the phsical demands of the sport prohibit participation in both at the highest level). And in evaluating GOAT, etc., nobody ever mentions his accomplishments on the doubles court?
But still...why does everyone rush to say that Fed/Nads/Cvac/Muzza would be great doubles players? Their volleys aren't even close to being as effective as Leander Paes', even at 40 years old. Not to say that they wouldn't have become great had they played a little more dubs, but the doubles game is predicated on three shots: serve, return of serve, and volley. In looking at the big four, each one of them has a hole in their games when it comes to doubles play. The BBs don't.
As doubles has always been a competely different game played by a different set of players, I believe the BBs are certainly doubles GOATs.