In other contexts a minute quantity of a forbidden substance can be disregarded. But the law of chametz is b’mashehu. The tiniest amount matters.
Unfair? Impracticable? Not at all. Either you remove all chametz or you don’t, and if you don’t, you have transgressed against the festival laws.
The reason is not only practical but symbolic. There are some things in life which are absolute. No relativism, no “it all depends”. There can be no bargaining or negotiating with some things, which remain wrong however one seeks to justify them.
That’s why halachah allows an emergency to override Shabbat or kashrut, saying pikku’ach nefesh docheh – “the saving of life must prevail”… but not when it comes to murder, idolatry or adultery. In no circumstances is cold-blooded murder, blatant idolatry or deliberate adultery permitted.
The law is b’mashehu. Just as a woman cannot be just a little bit pregnant, so there cannot be a modicum of murder, an iota of idolatry, an infinitesimal amount of adultery. Some rules brook no concessions.