> does not make any reference to costs or benefits to other users of the web platform

> arguments for are being presented publicly in that thread while those against are being discussed elsewhere (perhaps at Google?)

It's right there at the end of the highlighted comment:

"we would love to [...] replace them with something that generates less security bugs. Increasing the capabilities of XML in the browser runs counter to that goal."

So security bugs in XML seems to be the main issue?