The SCOTUS ruling was not based on legal precedent or thoughtful debate between conservative and liberal justices but rather an explicit attempt to shield future far-right presidents. Given the court's super-majority of pro-Trump justices, coupled with its current status of lifetime terms and lack of an ethics code, the only way to reign in their power and that of their friends in government is to codify a new law banning such ludicrous legal theories.
The Democrats are projecting their nefarious goals with respect to SCOTUS onto the Republican Party. The court's ruling was a very mild one that simply asserted the right of presidents — and not just Trump, but all presidents from either party — to govern without fear of being prosecuted by their opponents after leaving office. The Democrats don't want to reform the court—they want to completely redesign it.