This is easy to demonstrate: just look at the apparatus of the Nestle-Aland text (or the tc notes in the NET Bible) and you will see that the critical text had broad circulation, and early. I think the way the point is usually framed is this: The earliest MSS all come from Egypt, and therefore they do not represent the whole Mediterranean region. But there are severe problems with this thesis.
(a) It assumes that the MSS that were discovered in Egypt were produced in Egypt. But that is not only an unproven assumption, it is almost surely wrong in many cases. For example, Kurt Aland notes that there was no large church in Egypt sufficient to produce MSS in a scriptorium until the second half of the second century. Yet, we have evidence that the so-called Alexandrian text existed much earlier than that (e.g., Aleph and B agreements; P75 and B agreements; early patristic writers who quote the Alexandrian text, etc.). This must almost surely mean that the Alexandrian text, though it derives its name from the leading city of Egypt in the ancient world, did not originate there.
(b) The assumption also is that Egyptian provenance = Alexandrian texttype. However, this is demonstrably incorrect. The papyri which come from Egypt belong to the Alexandrian (both primary and secondary Alexandrian), Western, and even Caesarean texts. None of the early papyri--indeed, none of the papyri of any age as far as I am aware--belong to the Byzantine texttype. Now, how can we account for this situation? Surely it must indicate that because of the dry climate, Egypt was a repository of early MSS from all over the Roman empire. To be sure, the predominant text of Egypt would be in the majority, but MSS of other texttypes would also be expected to show up, as indeed they have. All except the Byzantine text.
(c) Finally, we really do not know where most of our MSS came from. For example, Vaticanus was discovered in the Vatican Library in the 1400s when the first catalog appeared. No one knows where it came from. Sinaiticus was found at St. Catherine's Monastery at Mt. Sinai. But it surely did not originate there, since the MS is 200 years OLDER than the monastery! There are so many problems with the facile solutions of the MT crowd that the whole house of cards begins to crumble.
By the way, I used to be largely persuaded by their arguments. And I held to that view tenaciously for 17 years. But after I did some serious investigation with an open mind, I changed my views. The evidence is what did it for me.