Ehrman on the NT text

Here's an exchange I recently had with a noted NT scholar:

As you know, a stock argument in Christian apologetics is to stress how well the text of the NT is attested compared to ancient writings generally. However, way back in 2005, Bart Ehrman produced an argument which, on the face of it, cuts the ground right out from under that appeal. And I've seen him repeat that argument in debates. He said:

Suppose, though, that the scribe got all the words 100 percent correct. If multiple copies of the letter went out, can we be sure that all the copies were also 100 percent correct? It is possible, at least, that even if they were all copied in Paul's presence, a word or two here or there got changed in one or the other of the copies. If so, what if only one of the copies served as the copy from which all subsequent copies were made — then in the first century, into the second century and the third century, and so on? In that case, the oldest copy that provided the basis for all subsequent copies of the letter was not exactly what Paul wrote, or wanted to write.  

Once the copy is in circulation — that is, once it arrives at its destination in one of the towns of Galatia — it, of course, gets copied, and mistakes get made. Sometimes scribes might intentionally change the text; sometimes accidents happen. These mistake-ridden copies get copied; and the mistake-ridden copies of the copies get copied; and so on, down the line. Somewhere in the midst of all this, the original copy (or each of the original copies) ends up getting lost, or worn out, or destroyed. At some point, it is no longer possible to compare a copy with the original to make sure it is "correct," even if someone has the bright idea of doing so.  

Suppose that after the original manuscript of a text was produced, two copies were made of it, which we may call A and B. These two copies, of course, will differ from each other in some ways — possibly major and probably minor. Now suppose that A was copied by one other scribe, but B was copied by fifty scribes. Then the original manuscript, along with copies A and B, were lost, so that all that remains in the textual tradition are the fifty-one second-generation copies, one made from A and fifty made from B. If a reading found in the fifty manuscripts (from B) differs from a reading found in the one (from A), is the former necessarily more likely to be the original reading? No, not at all — even though by counting noses, it is found in fifty times as many witnesses. In fact, the ultimate difference in support for that reading is not fifty manuscripts to one. It is a difference of one to one (A against B). The mere question of numbers of manuscripts supporting one reading over another, therefore, is not particularly germane to the question of which reading in our surviving manuscripts represents the original (or oldest) form of the text. B. Ehrman, Misquoting Jesus (HarperCollins, 2005), 59, 128-129.

In my observation, I haven't seen Christian apologists adapt to that objection. They keep using the same appeal to raw numbers. It seems to me that there are several basic problems with Ehrman's argument, but I'd like your opinion on two related problems:

i) Ehrman's argument is hypothetical. But how realistic is that scenario? To my knowledge, Christians in the early church were highly motivated to copy the Gospels and other NT documents for personal use and general distribution. So what are the odds that all our extant MSS of the Matthew, Mark, Luke, John, or 1 Corinthians trace back to a single scribal exemplar? Wouldn't we expect our extant MSS to issue from multiple, independent streams of transmission?

ii) Over and above the abstract probabilities, can we tell, by comparing extant MSS of, say, Mark, whether they all trace back to a single scribal exemplar, or do they have dissimilarities which evidence different text types? Do they have kinds of dissimilarities which evidence different underlying exemplars? 

To which he responded:

First, Ehrman's argument only cuts the ground from under those who must have a 100% accurately-preserved copy of the autograph.  If you're willing to settle for a little less than that (and, really, you have no choice), then his argument is impotent.  The facts remain that we have more copies of NT writings than for any other ancient texts, and that we have copies closer in date to the composition of their texts than for practically any other ancient literary text.  So, we're in much better shape for doing NT textual criticism than for any other such task.

But, yes, Bart's scenario is probably a bit oversimplified.  It's as, or more, likely that the Gospels were immediately copied multiple times, from these copies more made thereafter.  Now, on the one hand, every copying is a possible occasion for errors and intentional "improvements".  So, multiple copyings = a wider scope for such things.

On the other hand, multiple and early copies mean that we have more of the evidence needed to detect such accidental and even intentional changes, and so correct them.