Intermingling of that wish is permitted with that which is forbidden - in complex ways. For example, baking bread with wood from an orlah tree. Or hekdesh. Which leads to issues of both me'ilah and the item losing its hekdesh status. But is burning different? The implications for the ashes that remain are telling. The prohibitions with regard to hekdesh go beyond the usual food prohibition. Also - more on the Gemara's logic and the kal va-chomer. Namely, a kal va-chomer doesn't work if the end results end up being lenient. And this is used to knock down R. Yehudah's position that chametz must be burned (as compared to other means of destruction), but at the end of the day, this position that sounds more stringent could leave you with chametz in your house over Pesach.