The problem is that all of these autofillers are already way too complex, because almost no one uses the optimal markup (adding the attribute autocomplete="cc-exp-year", in this case)—almost no one has even heard of the proper autocomplete markup here (I remember being in a conference room with two or three hundred other web developers a couple of years back, and the speaker asked who knew about autocomplete="new-password" and the likes; only three of us raised our hands: I and my coworker, and one other).

They’re already complex enough that it’s a disaster trying to figure anything out. You say it wouldn’t be difficult to catch this, but either it’ll be a special case finely tuned for this particular site, or it’ll break another site, causing expiry year to no longer be filled out where previously it was and should be.