Well, that certainly rules out script kiddies and other non-state level threats but what about those state level actors? If a government with a blank check decides it wants to attack the network, would the fee really be enough to stop it?
Discussion
Governments can't have blank checks. They act like they do, but all they can do is print money really, and to attack they have to spend it all on BTC or XMR just to pay miners. The money devalues as they do it, and eventually to a point nobody wants to give them real BTC or XMR for their paper. They could confiscate people's stuff and sell it to other countries and keep it going a little longer, but that's destruction of their economy, ultimately they hurt themselves more than they hurt the network.