The Lord shall shave with the great razor Heb. (שְּׂכִירָה) , comp. (Jer. 46:21) “Also its officers (שְׂכִירֶיהָ) in its midst,” which Jonathan renders: its great ones.
on the other side of the river Of those who dwell on the other side of the river, and of which of those dwellers? The king of Assyria, the head He will shave and the hair of the legs. Since it is in the construct state, it is voweled with a ‘pattach,’ (שַׂעַר) instead of (שֵׂעָר).
shall be entirely removed Will be destroyed. The shaving is the slaying, and the razor is the sword.
the head This symbolizes the king.
the legs [This symbolizes] his camps [from Jonathan].
the beard [This symbolizes] the governors [from Jonathan]. But our Rabbis said that this literally refers to shaving, and the removal of the beard is by singeing it with fire. “The beard” refers to the beard of Sennacherib, as is found in the Aggadah of the chapter entitled, ‘Chelek.’
It is obvious that Rashi understand the "pshat" to be allegorical in this verse:
Peshat explanations are recognized in many more instances by their appearance next to non-plain comments…The non-plain sense also appears, in the vast majority of instances, linked with its own terminology. In an aggadic context one may also find אגדה (an Aggadah or מדרש אגדה (an aggadic Midrash)…Not infrequently Rashi’s Peshat, though terminologically still undefined, contains a reference to it’s source. In the majority of such instances the source is the Aramaic version of the masoretic text, i.e. Onqelos on the Pentateuch and Jonathan on the Prophets. (Peshat and Derash in the exegesis of Rashi,By Benjamin Gelles, pages 20-23)
We must wonder, if you pardon my co-opting the term, what is bothering Rashi? Most of us would be inclined to understand this verse as allegorical as well, but we tend to think of God as non-corporeal. If Rashi was a corporealist it is not at all clear that he should present the peshat as allegorical, especially when Chazal seem to take it literally. After all Rabbi SIifkin argued regarding Gen. 11:5 which describes God as "descending" to "see" the Tower of Babel, "Rashi is citing the Midrash, which may well have understood Scripture non-literally, but Rashi does not show any concern (as does Onkelos) that one may interpret it literally." (page 96). In either instance God is, as it were, coming to Earth to perform functions which are, or in a manner which is, associated in humans. In our verse, however Rashi does not understand the more corporealist view as pshat even though there is traditional "basis" for doing so. Furthermore, even when citing the midrash found in San. 95b-96a he omits the most corporealist language. This would be entirely consistent with him holding a position similar to the Ra'avad that while he does not accept that God is corporeal, he may not be overly concerned that simple people have such a [mis]conception.
Insofar as I have not dealt with all of Rabbi SIifkin's arguments I do not presume to suggest I have refuted his position (although I'm inclined to believe the passage cited by R. Waxman seriously undermines it). What I believe I have demonstrated is that his initial line of reasoning is flawed and does not support his position. It is an argument from silence based on incomplete research. His arguments for why we should expect Rashi to comment are not conclusive enough to escape this fallacy. Even if his other lines of evidence are solid (or conclusive for that matter) this one is in no way מצטרף such that his position is stronger than the other lines of evidence are on their own.