These days I almost take it for granted that somebody isn't going to read my email, or won't read it thoroughly, or will read it but will fail to acknowledge it. One can use this to their advantage if they want to skirt a hard "no" but as you said, it may backfire.

And the "boss" may have a point: relying on them to read, understand, and acknowledge your email, especially when it's important, is somewhat disingenuous. At the very least one has an obligation to confirm that the recipient actually read and understood what was sent, before taking the default action.