Amazing how "the American government is a system of checks and balances" has evolved into "the check and balance is the Supreme Court, which can strike down anything at will, and you mustn't even discuss the Court, itself, having checks and balances upon it, since that would be tacky".
I can't stress enough: there's no law saying you can't add justices whenever. There's no law saying a Supreme Court justice has to have legal training at all, even. You could add a dozen explicitly partisan hacks onto the court to vote a certain way, and it would be 100% legal. Or you could, you know, add one or two judges. We're letting all this happen since it would totally make us feel icky if we did that, and also we've decided that "checks and balances" doesn't mean a system on all three branches, but merely one branch having total control of the other two with no input allowed except voting for new additions