> to dismiss this as a profound advantage is to definitely sell the idea more than a little short.

Nah not really what he's saying. He's saying that if you throw out all the security affordances provided by page tables and virtual memory, it outweighs the "profound advantage" (which as he mentions, is arguable anyway since user/kernel context switch is a negligible cost in most modern systems).

You're selling a great deal in order to buy not much. It's a poor tradeoff.