And the 14th Amendment applied, among other things, the Bill Of Rights - and hence the 2nd Amendment - to the states as well. If SCOTUS decrees the 14th doesn't apply after all (per the 2nd), then the 13th doesn't apply either, and pretty much the whole "Constitution" contract gets flushed.
The judicially created incorporation doctrine was unknown when the 14th Amendment was ratified. Even the incorporation of the 1st Amendment (i.e., pornography is free speech, school prayer isn't) wasn't invented until 1925.