“Assume ultimate legislating ability”- Unless you are whining about Marbury v Madison, what on earth are you talking about? SCOTUS doesn’t write laws, they rule on the permissibility of (a small fraction) of them.
“Impractical supermajorities”
Did you just discover what checks and balances are? One should want supermajorities because you don’t want laws based on shaky public support. Do we really think the cycle of each president overturning the previous presidents policy is practical?