Then it would have ceased being “Classic” and become “Naxx-Classic”. Which would have the same results as Classic WoW has had with the 2019 “Classic WoW” and the eventual “TBC-Classic”/“Wrath-Classic”/“Cata-Classic”/ and the upcoming “MoP-Classic”. Only I doubt they would continue to have a separate mode for the Vanilla “Classic Hearthstone” and the expansion ones.
This is pure conjecture on my part here, but I think it to be a logical thought process about this potentiality. It would likely have just become like the continuing Classic-Re-release timeline mode and left the original vanilla behind in the dust. But considering they probably could see the lackluster sales of the classic packs it was probably deemed not a viable path forward to try and get old sets to sell again like they successfully have done with WoW and its classic timeline re-releases alongside retail.