Smart, sorry, I'd missed that from your original post. But it seems like V. Buterin's argument from the link in the OP still applies:
"This seems weak, but in reality it isnt; we know that in the case of Bitcoin, once the currency supply stops increasing mining will rely solely on transaction fees, and the mechanics are exactly the same (since the amount that the network will spend on mining will roughly correspond to the total number of txfees being sent in); hence, fee-based TaPoS is in this regard at least as secure as fee-only PoW mining."
(The premise of this argument is a standard "free entry" condition. Entry continues until returns are driven down to zero.)
I will have to think if an improved confirmation condition robust to multiple double spends could be thought up.