The solution to that is easy: do not use tabs.

That is not the approach taken by any system that I know of that uses semantic whitespace. Tabs are meaningful in Python and make, and in fact required in make.

Haskell and Nim are both indentation-sensitive languages that only allow spaces as indentation.

That’s a design flaw of ASCII inherited by UNICODE not an argument against using indentation to portray hierarchy. I’m sure there are alternative UNICODE characters which render almost indistinguishable from “(“ also.