I would be a bit more specific here because:
- Luke is the only Gospel that actually claims authorship, and Luke specifically says he was not an eyewitness, but was writing what was handed down from eye witnesses. I don’t think Christians, generally, specifically claim that Matthew had to have penned the Gospel of Matthew, for instance, only that it was his account.
- You may need to cite specific experts here, because it seems like you are claiming that the Gospels couldn’t be the accounts of eye witnesses. My understanding of the field (and I am not an expert at all, hence I would be grateful for citations or specifics) is just that there are questions as to specific authorship, but that it was widely recognized that the trend was towards earlier authorship (at least within a couple generations of the actual events) not away from it.