Sampras was not at peak in 2000, Federer not at peak in 2012, Borg probably not at peak in 1976, Laver not at peak in 1961-62 (albeit that Rosewall/Hoad/Gonzales could not participate).
And none of these men has a 0-5 record in a particular tournament final.
By the way, if tennis had been Open in the 1950s, Rosewall would likely not even have reached the two Wimbledon finals in that decade - he was fortunate that stronger competitors were not around, to allow him to reach two finals! Contrast this with Laver who, as well as dominating elsewhere, won two Wimbledons before turning pro.
Newcombe is the only true great in that list.
If Borg had never played the US Open, people would have said "he would have had great chances every year, especially when it was played on clay". Point is, he did play, and never won it.
I actually don't find it likely that Rosewall would ever have got over his hump and won Wimbledon. I expect he would have consistently got to the latter stages and been beaten by Gonzales, Hoad and Laver.
A tough challenge for the champion yes, but a champion himself? I'm not so sure.
Very unlikely that the top 2 players of all time played in exactly the same era , and hail from the same nation.
And you seriously think that nobody who began their career after the mid-1950s could be the GOAT? I humbly suggest that, as a 64 year old man, you are in thrall of the idols of your youth and cannot appreciate the greats of later eras (Borg, Sampras, Federer, Nadal) as much as they deserve.
And finally, once more,
you overrate Rosewall.