This reminds me a little about arguments in other fields, specifically in boxing (in which I am even less of an expert than I am in table tennis). But I do remember arguments about Ali v Marciano, which were eventually "settled" by an actual "fight" between them (1969?) It was of course not a real fight, but a staged one. In the US, the film of the fight showed Marciano winning, and in Europe, we saw Ali win. Later, Marciano is reported to have told a sports writer that he knew as soon as Ali stepped into the ring, he (Rocky) would not have stood a chance against him even in his prime. " Too big, too quick...He had the fastest hands I've ever seen".
I have no great insight into everyone who ever played the game, but if I had to bet the house on a match between anyone and Ma Long, assuming that all other things were equalised (ie, Ma Long has gone back in time and been allowed to practise with whatever conditions applied, or vice versa for the other guy), I'd go with Ma Long, simply because of his TT brain. He seems to have had the analytical skill of an older and wiser LGL transplanted into his head, where it meshes with the best all-round physical game of ML.
As it stands, you can only play the opponents you meet. I think it is a logical fallacy to suggest that someone is not the best because he beats everyone, and someone from the past is the best because other people won some titles too in that time. We are in trouble comparing one to one, without comparing the whole current TT playing community against the former TT playing community, IYSWIM.
BTW, leaving aside questions about the GOAT, if there had been no Ma Long, what would table tennis rankings look like today?