Actually, real text crits don’t hang on Codex Sinaiticus either. They compare across a wide range.
The TR as a whole is not based on only one Greek text, but was based on a relatively few number of relatively late copies of texts which themselves aren’t necessarily all that late in terms of their families. (This was done on purpose by the TR compiler, Erasmus, who was racing to produce the first standardized critical-comparison Greek text. He got done first by using fewer and later texts; after which his publisher squeezed the other guy out by aggressive marketing: IT’S THE FREAKING TEXTUS RECEPTUS, THE ORIGINAL RECEIVED TEXT OF THE NEW TESTAMENT BY GOD AMEN!!! {thoom} 
)
However, Erasmus did have only one Greek text of RevJohn available to him at the time, although (following text-crit principles) he did weigh in some extra-canonical references. (This, by the way, is why he had to base the last six verses of RevJohn from Jerome’s Latin Vulgate–they were missing from his one Greek RevJohn text, and were the earliest source he had for them. This is ironic when one considers how nukey the “Speciality Interests” guy gets, concerning Jerome’s Vulgate… )
Text critics try to keep in mind many various reasons for why readings may vary, and one criteria is that the harder reading should be inductively favored (though not exclusively so–just weighed in the estimate) precisely because people have a tendency to emend and gloss the text into ‘easier’ readings.
The “Speciality Interests” guy is a good example of that. He can’t think of any reason for why anyone would emend the text to read “wash their robes”, other than a general impression of ‘spiritual corruption’ in the church starting in the 4th century when Constantine became Emperor, and a guess that Alexandrian “philosophers” wanted to get away from the concept of doing what the Lord commands. Logically, though, that kind of thinking is exactly how the original text could have been easily emended to read “do His commandments”, which in Greek sounds kind of similar when read out loud.
It should be noted that, strictly speaking, there is no conceptual difference between washing the robes and doing His commandments (aside from the former more obviously avoiding an impression of earning-salvation-by-works) in the application of 22:14. Either way, the Spirit is exhorting those sinners still outside the New Jerusalem, having suffered the lake of fire judgment after the general resurrection, to repent of the sins that they are still insisting on holding to, and accept the gracious cleaning of Christ, allowing them to enter into the city and be healed. The phrase “washing the robes” just makes this more topically connected through to verse 17. True, it also helps emphasize the gracious salvation offered to those outside the New Jerusalem in a fashion universalists especially would appreciate, but to say the least Athanasius never showed any signs of accepting universalism (that I’ve ever heard of anyway. 
But then, see below… ) Nor does the metaphor actually undermine the personal responsibility of the sinners to repent and act to accept the gracious salvation of God: otherwise, there would be no point for the Spirit to be exhorting the sinners to go down to the river and slake their thirst and wash and so obtain permission to enter the City!
Metzger’s 2nd edition commentary on the USB’s Greek New Testament (which is basically also the Nestle/Aland compilation, but with less exhaustive “apparatus” listing all known textual variants), reporting the rationales of the editorial committee, has this for 22:14:
{plunontes tas storas auto_n} (rinsing the robes of them), is supported by the two (older) unical texts Aleph (that’s Sinaiticus, which has special connections to Hebrew/Aramaic, which is why it’s identified among text critics by the Hebrew letter ‘A’ 4th cent) and A (that’s Alexandricus, 6th cent), plus about 15 miniscules (typically late, but with one going back to maybe the late 9th century, and a couple of 10th century), an Old Italian manuscript from the 9th century, Jerome’s Vulgate (4th century–ironic because the Textus Receptus beloved by the SpecInt guy had to go back to Jerome’s Vulgate for the final six verses of RevJohn ), the Sahidic Coptic (from the 3rd century!–also ironic, since the SpecInt guy thinks the Coptic only references the “do the commands of him” variant), Ethiopic (very early 6th cent), and other (typically later) refs. They mention Pseduo-Athanasius as a minor variant phrasing, but have no idea when to date him. They do not regard the real Athanasius as having written anything on RevJohn. (SpecInt is referring to a text universally considered to be pseudonymous by scholars and rather late.)
{poiountes tas entolas autou} (doing the precepts of him), supported by unical text 046 (one of the Byzantine family, 6th cent–a text that only features RevJohn, incidentally, and is missing the final six verses, less incidentally), most miniscules (typically late, but a few going back to 10th cent), an Old Italian manuscript (13th cent), two versions of the Syriac Peshitta (6th and 7th century respectively–obviously a big point of SpecInt is to date the Peshitta much earlier and as not having been translated from Greek), the Bohairic Coptic (3rd century), Armenian texts (back to 5th cent, featuring a minor variant of same idea), and other (typically later) refs. Plus Tertullian (Latin, sometime after early-mid 3rd cent), Cyprian (Latin, mid 3rd cent.), Andrew of Caesarea (Greek, late 6th/early 7th cent), Caesarius of Arles (Latin, middle 6th cent–not actually quoting the phase but clearly referring to it), and Beatus (Latin, tail end of 8th century–not actually quoting phrase but clearly referring to it). The 4th edition of the UBS doesn’t mention Tertonius being weighed in anywhere for anything, but SpecInt says he used this phrase in late 4th century. No idea what language.
The dating and family spread is about comparable either way, enough so that the UBS editors don’t seem to have factored date or family spread in as a weight in the final analysis. They appealed to several other factors:
1.) the two phrases sound somewhat similar when read out loud in Greek.
2.) the author of RevJohn uses the phrase “keeping the precepts” twice earlier in RevJohn (12:17; 14:12). Even though “keeping” isn’t at all the same word as the rather complex word for “doing” (used in the variant being debated), the concept is pretty close (and the term for precepts is identical). Considering that just previously in the chapter (verse 12), Jesus was represented as warning (and promising) that He is coming quickly and bringing His wage with Him, in order to pay each one according to his work, it makes more sense for scribes to gloss “rinsing their robes” to read something similar-sounding in Greek that fits this idea, than for scribes to change this back over the other way. (A text crit principle is that a reading between two variants which is harder to explain by scribal alteration, should be given preferential weight.)
3.) the committee thought “the prepossessions of the scribes” would tend to favor changing “rinsing their robes” to something more doctrinally appropriate, too. (Same principle as the second rationale, but applied a different way.)
Incidentally, I don’t know where SpecInt is getting the idea that “most Greek texts” (including Vaticanus) read “the word of the life” or “the word of life”, in verse 14, but all the Greek texts I have access to (including Green’s 3rd edition of the Textus Receptus!–plus the King James Version!) uniformly read “tree of life”–and the UBS doesn’t mention any variant there at all. Considering that the tree of life is certainly a figure for Christ, the Word of Life, the variation need not be doctrinally significant; but I suspect this particular variant is verrrrrrrrry late.