Based on feedback from people at EoFF, on reddit and metacritic and elsewhere, I'd say that FFXV has done very well with customers and that the high sales reflects this well.

"If it were a better game, it would sell more" can literally be said about every single game in the entire world, including every other game in the Final Fantasy series.

The feedback from the public has been better than that of XII and XIII, as far as I can tell. I agree more than a handful said it was not good, but again, that can be said about every other game in the FF series.