Yeah, fans are biased - thus the propaganda. But 'ware your own as well, my friend! You keep accusing Federer fans of propaganda, of positing certain views in order to prop up their hero, but are you not doing the same to some degree?
Anyhow, to me the term "GOAT" can only ever mean "so far" - without that it is meaningless as the future hasn't happened yet, so therefore I don't think it is necessary to add it; thus GOAT inherently means "so far" to anyone using it. So the future thing is a bit of a red herring, imo, as no one ever uses it that way.
Note that I also mentioned Rafa as the greatest specialist, which you conveniently ignored! Rafa is clearly the greatest clay court player of all time, right? I would also say he's the greatest specialist of any kind. But as Fiero rightly points out, he doesn't have the same degree of dominance on hard and grass courts (although still an excellent record).
I also disagree with your reasoning as to why there can never be a GOAT. Certainly what you say makes it more complex, but what we can do is compare relative dominance across eras. How dominant was Sampras during his era vs. Federer in his? What about Borg vs. Nadal?
Part of the problem is that there is very little good statistical analysis of tennis unlike baseball and, to a lesser degree, other sports. Numerous systems have been designed for baseball--the ultimate stat fetishism sport--to compare players across eras, and the better (and more recent) ones adjust for context. Not just era, but ballpark and other factors. But it is still evolving and, like Zeno's arrow, will never reach its goal of the Perfect System.
There is always going to be a subjective gap, but I'm comfortable with that. In fact, that allows for debate. But tennis needs more objectivity, in my opinion, more statistical analysis. Hopefully some enterprising nerd will come along and get the work done!
