The generational thing is something I was chatting to someone at work about yesterday. If you had a time machine and went back and got prime Pele, and chucked him in today's Brazil side, would he be any good? Possibly not. But if Pele was born in 2002, and had all the information, training, nutritional advice, etc, that today's players have would he become the goat? Maybe.
I don't subscribe to the "have to have seen him" argument. We have enough information at our disposal, including the accounts of people who did see these players, to have an informed opinion of the players of the past. Hughie Ferguson scored almost 300 goals, and McFadyen scored 52 in a season, that alone is enough to tell me they should be right up there in the reckoning.
The stats do give defenders and goalies a raw deal though.