particle は - ‥ができないと vs ‥はできないと

Every evening, we play the かたづけ怪獣 song, where they sing:

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！

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:

If you can't clean up, the cleanup monster will come!

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

If you can't even clean up, the cleanup monster will come!

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

Answer

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,

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