We'd probably have to talk about it IRL; the question you're asking kind of implies you're coming at the whole thing from the wrong direction, and so any answer I give would be unsatisfying unless I could somehow transfer the entire philosophy in my head to you.
The short answer is that what you're asking doesn't really make sense to me. I think of the lifetime of state as analogous to member variables of a class. In the same way that member variables in a class are always visible to the entire class, a state variable in React is always visible to the entire component. You wouldn't want it to be scoped to an if block any more than you'd want a member variable to be scoped to an if block.
Maybe now you're thinking "well, that's limited in an annoying way, and Svelte is better". Maybe. But I suspect for any problem that exists that you think needs conditionally-scoped state, there's a nice clean solution in React. That's what I suspect, anyways. But I very rarely run into problems in React that I can't express in nice, clean code.
