Roglic surely is better right now than Sastre... still I think it's just very hard to look past the Tour as a king-maker. I get a bit of what Rackham says, that it's not just the numbers, but if people look back on a "legacy" that's exactly what they will look at, I suppose. But Roglic has the advantage that his career isn't over yet.
I think that even if Roglic's career ended now, he's ahead of Sastre. Sastre just didn't win much. He wasn't a one day racer, and I don't even think he won any one week races (happy to be advised otherwise though). He was sort of a little bit better than Zubeldia (and also admittedly far more attacking), and happened to win a Tour (the TOURRRR!).
But Roglic is a truly dominant rider of the peloton. There's all the one week stage races that he wins (plus the other ones that he almost does), plus he's won LBL, and Olympic TT gold, and 3 Vuelta's.