‥ができないと vs ‥はできないと


Every evening, we play the
[かたづけ怪獣](https://www.youtube.com/watch?v=xqJn455XMsg) song, where they sing:

[code]

    おかたづけができないと
    あらあら、大変 (x3)
    おかたづけ怪獣が出て来るぞ！
    
[/code]

Now I've been grappling with this language long enough that ‥ができないと sounds
natural in this context, while ‥はできないと sounds off, but I couldn't put my
finger on _why_. My wife, a native speaker, pondered this for a moment and
then theorized that は would be a simple statement of fact:

[code]

    If you can't clean up, the cleanup monster will come!
    
[/code]

While が implies that, while there may be lots of things you can't do, surely
you can _at least_ clean up:

[code]

    If you can't even clean up, the cleanup monster will come!
    
[/code]

I buy the first half of that explanation, but the second seems fuzzy. Is she
right, and is there a more general rule here?




I think it's the opposite? が simply states the fact, while は would imply "at
least". I'm a native speaker myself.

For example,

[code]

    銀メダルが取れた
      -> I got the silver medal
    銀メダルは取れた
      -> I got at least the silver medal (but not the gold medal)
    
    銀メダルが取れないと帰れない
      -> If I don't get the silver medal, I can't return
    銀メダルは取れないと帰れない
      -> If I don't even get the silver medal, I can't return
    
[/code]




I think it's the opposite? が simply states the fact, while は would imply "at
least". I'm a native speaker myself.

For example,

[code]

    銀メダルが取れた
      -> I got the silver medal
    銀メダルは取れた
      -> I got at least the silver medal (but not the gold medal)
    
    銀メダルが取れないと帰れない
      -> If I don't get the silver medal, I can't return
    銀メダルは取れないと帰れない
      -> If I don't even get the silver medal, I can't return
    
[/code]

