So how would such a ruling adversely affect the RKBA?
Do you want 5 USSC justices ruling that "arms" in the second amendment does not include handguns? (not far fetched -- the 7th Circuit said exactly that in Quilici v. Village of Morton Grove -- "Under the controlling authority of Miller we conclude that the right to keep and bear handguns is not guaranteed by the second amendment"). That ruling would then apply to all states.
States with the RKBA would still have it.
How would such a USSC decision make the RKBA less secure?
The same way the USSC ruling on nude dancing made state laws against it less secure. All it takes is one lawsuit to overturn state laws. Which there were.
And there are plenty of gun grabbers out there with beau coup funding to challenge state handgun laws, knowing in advance that they have the green light from the USSC.
State handgun laws would topple like dominoes.