One problem is that this basically undermines one of the three branches of federal government, rolling the Supreme Court into the legislative branch, and allowing a ton of misbehavior from that branch to oversee its own behaviors.
It allows the legislative branch to pass unconstitutional laws by setting up the appeals courts to elect Supreme Court Justices that it favors.
It sets up a conflict of interests and undermines checks and balances in exactly the sort of way that the US government structure was avoiding.