and I suppose I shouldn't assume that if you haven't replied to Tom, it means you don't have any bone to pick with his post.

But honestly, you seem to be rather inconsistent here, or just intentionally misunderstanding me. Tom writes a post complaining about a construction he encounters a lot, which is obviously said by a fair amount of people, and which he considers wrong, with some justification on the one hand from grammar rules, and on the other from history and the alternative being far more widely used. If I then reply by citing two examples of things that - to my knowledge - every or nearly every speaker of English would say that way, but which equally violate the grammar rules about when to use a personal pronoun in the nominative and when in the accusative, it seems rather obvious to me that "ungrammatical" refers strictly to the grammatical rules (which, incidentally, besides being the obvious interpretation, is also how Merriam-Webster defines it: "not following rules of grammar" - it doesn't say anything about being considered correct by native speakers or not).

If you then take the strong anti-prescriptivism stance that most linguists will take, and don't allow something to be called wrong based on grammar rules if it's sufficiently widespread, that leaves little room to support Tom's original point - the only argument left then is arguing that "between you and I" is not (yet) sufficiently widespread. Or you could disagree as much with Tom's post as with mine, as I said I shouldn't make assumptions about that.

Lastly, I think the smiley in my title and the part about one being unlikely to get much agreement with such a change make rather clear that changing how people speak English was never my intention.