Yes, clearly. I just meant "… in a vacuum" or "… with the implicit understanding there is some _true_ grammar". In one sense, yes, utterances/productions can be

Yes, clearly. I just meant "… in a vacuum" or "… with the implicit understanding there is some _true_ grammar". In one sense, yes, utterances/productions can be grammatically incorrect with regard to a particular grammar under discussion, but if those utterances/productions can be said to be a part of that grammar, eventually the grammar needs to change to accurately describe the state of the world. (Ultimately I have to think some of this comes down to "misperformance" where the speaker _knows_ their production is invalid. If enough speakers "misperform" but begin to hold the idea that these speech acts are not actually invalid, is that what tips the scales in favor of changing a grammar?)