Bart Ehrman destroys Christianity in under 12 minutes.

[Aractus]

In Luke 11:4, the phrase αλλα ρυσαι ημας απο του πονηρου (but deliver us from evil) is omitted. The omission of this phrase is also supported by the following manuscripts: Sinaiticus, Vaticanus, Codex Regius, f1, 700, vg, syrs, copsa, bo, arm, geo.[13]

So, even near the end of the 2nd Century, the Lord's Prayer was not yet completely established!  Who knows the variants that were taking place prior to P75?!  That's Professor Ehrman's point -- no one really knows the true words of Jesus.

Well we do know because scholarship tells us. They are certain we have ~98% the original wording of the gospels (with meaning at greater than 99.5%). That specific manuscript has its own specific anti-Semitic recension that are quite subtle, and are not found in later manuscripts. This fact alone proves that every other manuscript of Luke and John that exists today comes from a different textual path, and that is despite this being an Alexandrian text-type manuscript, the same as P66 which doesn't have the anti-Semitic recension.

We also have Papyrus 66 which is John and most scholars date it late 2nd/early 3rd century. There's at least one textual scholar who dated it to early-mid first century, and another who argues that it's a 4th century manuscript. Assuming we can take it to be a late 2nd/early 3rd century text, it corroborates the text of P75 whilst not having the anti-Semitic recension seen in it, thereby showing that textual variation was not systemic.

To put it another way, Bart's arguments play on the ignorance of his audience. His books are intended for an atheist/agnostic audience with little knowledge about NT scholarship. So he will exaggerate what a "textual variation" means. What he doesn't care to explain clearly is that Greek is a more fluid language than English, and for any one single Bible verse there could be dozens or more ways of writing the same sentence in Greek without every changing the meaning. One such example "Jesus Christ" vs "Christ Jesus". No matter which way it's written it has the same meaning, with Christ being a title (such as 'mr', 'sir', 'dr', etc). And then of course there's the Nomina Sacra, which adds yet two more ways to write "Christ Jesus". P66 and P75 both make use of the Nomina Sacra, which is why some scholars don't want to date P66 in particular too early, but regardless their discovery proved the early use of the Nomina Sacra which had previously been thought to have been introduced in the 3rd century. It has now been found they are used consistently in Christian writings, including non-scripture texts, that go all the way back to the second century - and perhaps even to the first century.

Basically the way that he talks about textual variants is inconsistent with scholarly thought. Yes there are many textual variants, but for the most part they are just different legitimate ways to write the same words or phrases in Greek, or to make use of a common abbreviation (as we do in English for terms such as "et cetera") or to make use of the Nomina Sacra or simply because the way the word is more commonly spelled in the fifth century was different to the way it was most commonly spelt in the 3rd century, and on and on the list goes on. Then you get scribal error (things misspelt, words omitted, etc) which adds another layer of textual variants. Between them they account for the vast majority of observed variants, with the actual variants that have a different meaning and where the original meaning cannot be known (such Revelation 13:18) being very rare. Take note of the part I emphasised, yes we have a few examples of later additions - but those have been identified and we know what the original form of the text said: for example we know that Mark ended at 16:8, we know that the Comma Johanneum was deliberately inserted into Greek for the first time in the sixteenth century, but we don't know whether the author of Revelation originally wrote that the number of the beast was 666 or 616. And today there's only one manuscript in existence to have the 616 variant, but it is early enough to cast doubt over the original language. Note the example I gave of course was Revelation, which as a text is not of much interest to critical scholars anyway. In your example, it may have been omitted in Luke - but it is found in Matthew. So we know exactly why it was later added in later manuscripts if you look up most modern translations you'll see that 'deliver us from evil' is not found in Luke 11:4 (with exceptions to those based on Textus Receptus).

New manuscripts are constantly being discovered, the now 'infamous' Mummy Masks have promised to revel many more early NT manuscripts on papyrus, including a near-complete copy of Corinthians. Bart himself said he was horrified that they are desecrating ancient artefacts to extract manuscripts, and that for me has ended the respect I once afforded Bart as a textual scholar. A textual scholar would be far more interested in recovering early papyrus NT manuscripts (which as you pointed out are rare) than in preserving ancient burial masks made from them. There are some archaeologists who might be horrified at the idea - yes - but NT scholars should be emphatically excited at the prospect of having more very early NT manuscripts from the second, third, and perhaps even first centuries.