People make mistakes now. That’s why you write a compiler to generate good error messages. 
Actually, your example would compile, but just might not be what you intended
def call
if x
if y
end
end
end
Look, with every new thing comes a learning period, especially if you’re used to the old thing. No one would be required to use this, so if people don’t like it they just won’t use it.
However, I know people coming from Python|Nim, and other whitespace languages, will love this. Once you grok it, it will soon become second nature.
And after time, even some old dogs will begin to learn some new tricks, to make their lives easier. 