Is "as [adjective] as [can/could] be" grammatical?
Solution 1:
Well, there's no God-given list of pieces of grammar that are "bad" vs pieces of grammar that are "good"-- just people who invent arbitrary reasons for labelling things as such and then other people who decide to agree with those reasons.
So, you've invented your arbitrary reason for deciding it's "bad" grammar. You just need to decide to agree with yourself and you're done...