Sometimes there’s nothing better to end a sentence with.
It is not a rule, only a tradition, that the end of a sentence is not the place to tack a preposition onto. The presumptive prohibition appears to have had its start in 1762, when clergyman Robert Lowth decided he didn’t like sentences that used prepositions to end with. In his A Short Introduction to English Grammar, Lowth opined that such was bad form. Pundits have ever since found final prepositions ideal for pouncing on.
Correct grammar is determined by usage, not by rule-makers. Even languages like French, which boast official rule-makers who delight in cranking out official rules, cannot escape that essential fact.
Consider, for a moment, how utterly nonsensical is the expression, Ain’t isn’t a word. I challenge you to read the definition of the word “word”* and then explain in what sense ain’t ain’t one.
*From Wikipedia: “A single distinct meaningful element of speech or writing, used with others (or sometimes alone) to form a sentence and typically shown with a space on either side when written or printed.”