"The Legislature can pass a new law tomorrow to change the entire situation and SCOTUS can only react."
As it did today, for example. Merely reacting.
Or as it did with the gay sodomy decision.
Or perhaps as the Massachussets Supreme Court did with gay marriage.
As it did with Roe v. Wade.
As it did with the decision finding the death penalty to be cruel and unusual punishment and unconstitutional.
And then when it decided, a few years later, that the death penalty was not cruel and unusual punishment.
As it did when it decided that no teenager can be executed no matter what.
All purely reactive?
That's the beauty of the American system. Nothing is purely this or that.