Really interesting write-up, and thanks for sharing your point of view.
I think you’re striking a cord here, but I don’t even think the whole point of Mobile First was purely about the increased amount of mobile accesses we had/have. As Luke Wroblewski states in his original article about Mobile First, https://www.lukew.com/ff/entry.asp?933, it also forces you to focus on content, which is what I considered the most important bit in there.
If you’re looking at the bigger picture, we’re always striving to improve our processes and use patterns whenever needed/fit for the purpose, to deliver better products faster and have a captivating user journey and user experience that satisfies the needs of all our users, whether they’re on the mobile or on the desktop.
Making a distinction between mobile, desktop, tablet, phablet, watch, whatnot, is somewhat pointless, as designers and developers, we should instead concentrate on content first rather than anything else (or even better, on the Content Strategy). If this leads to a different UI interpretation in certain situations, that’s only better for the end user.