The supreme court decides which cases they hear. If a decision in a lower court is appealed and the SC decides not to hear it then they have basically determined that justice was served in the lower court. So in effect they can do something by not doing anything. Plenty of 2A/RKBA cases have been appealed the SC and turned away.The judicial branch cant do anything until cases are brought before them. They dont get to just wake up one day and say " Hey, this law is unconstitutional I'm just going to override it". A complaint in the form of a lawsuit or appeal has to come forward.
Since the two justices appointed by Trump have been on the bench, the SC hasn't accepted any cases regarding state bans on semi-auto rifles. Basically, so far, the lower courts have had the final say.