No, I agree that it’s perfectly valid to challenge folk etymology. What has got me puzzled is Motcha’s emphatic confidence when his evidence really is the same kind of evidence as offered by Rosten and the others.
To (over-)simplify, if Motch’s proposition is – “I know people who speak Yiddish; therefore, I am right.” Then we can compare this proposition to the evidence offered by Rosten and the others. Can they also say “I know people who speak Yiddish; therefore, I am right”? I suspect that many of them can. Did Rosten know a lot of people who spoke Yiddish? (I’m guessing yes.) Did any of the writers of dictionaries know people who spoke Yiddish? (Okay, maybe less probable.) Did any of the Jewish-American entertainers know people who spoke Yiddish? (I’ll guess yes again.)
I’m not saying that means that Motch is wrong, but that the kind of argument he is offering and the manner in which he offers it doesn’t seem to quite jive. Now, the other evidence regarding the dates and stuff, that seems more concrete.
Furthermore, Motch started off by saying that at least some of his Yiddish-speaking associates were prolific swearers. Then he says, well, really, their style of swearing does not employ vulgarity. Well, so how is that evidence relevant when examining what is proposed as a vulgar term?
(As an aside, I think “schmuck” would qualify more as a vulgarity than a profanity – there doesn’t seem to be any reference to god.)