The math
The loss of Ruth Bader Ginsburg moved the Supreme Court to a conservative majority: there are now only three reliably liberal justices. Previously, one of the five conservative justices could swing a decision, but now a single convert to a more liberal argument would only create a tie. A tie upholds a lower court’s decision until a related case might make its way back up to the Supreme Court.
Part of Trump’s tax “cut” was the removal of the ACA related penalty for not buying into a health care plan. In 2012, Chief Justice Roberts voted in the 5-4 majority to uphold the ACA based on the reasoning that the penalty was “a constitutionally appropriate tax.” Without this tax, Roberts’s prior reasoning falls away, and the 5th Circuit used this to uphold a Texas judge’s ruling that the ACA was unconstitutional.
The Supreme Court of the United States (SCOTUS) scheduled arguments for November 10 over whether the ACA is constitutional.
Even if Justice Roberts continues to vote in favor of the ACA, with RBG gone from the court, his vote, at best, will create a tie. That would allow a lower court ruling against the ACA to stand; that ruling would be binding only on the 5th circuit, but it would stand as case law for other courts to rely on, until such time as a similar case was brought in front of the Supreme Court.