In your diagram of the simplistic deployment stack, I notice a distinct lack of layers labeled "Plack". Or is your overall point that "Plack" really just means "any arbitrary implementation of PSGI"?
Yes. We only happen to have one and it lives in Plack:: but it's just a PSGI implementation/harness; just like CGI.pm is a CGI spec implementation. I'm glad miyagawa chose to keep the names separate though I suppose both approaches have some points of confusion.