I bet you can go to block explorer and retrace your history, but have you ever tried to do it with someone else's history? It's hard! At every fork, you need to make a guess, and after just a few transactions it's difficult to have any confidence that you've followed their history correctly.

Now, if you were an investigator trying to follow the evidence for a major case or study, you could be meticulous about it. You could look at transaction types used, what time transactions were included in blocks, the fee-per-kb paid, and other data which could fingerprint transactions made by a particular person with a particular wallet program.

But against an idle spectator like myself, key rotation does a good job of muddying the waters enough to, as I said, "provide a reasonable degree of financial privacy"

Of course, I think the merge avoidance strategy will be really cool if payment processors and wallet software implement it, and I agree that we should always write about privacy when there's an effective solution available.