In contrast, the transition to "mobile web first" is already done in China, which I would argue has the largest monetizable mobile consumer market in the world (because it's not only for hip young people. Most if not all of the grandma generation in China are getting on the internet for the 1st time, and only through WeChat on smartphones). Any user of the WeChat app knows the astonishing volume of contents shared within that social app, and all of them are mobile centric, actually it's fair to say it's "mobile only" (if you load the same URL of a shared article in WeChat on a desktop browser it'll look aweful i.e. they are not responsive, and they don't care). Yet, this transition happened not because of WeChat pushed all content providers to adopt a certain standard, but done spontaneously by each content provider, on their own to figure out what looks good on WeChat. The need to make "mobile only" content comes from the desire to reach the huge user base in WeChat, which, unlike facebook, is a "mobile only" app.
The tech for making mobile friendly webpages are there for years. I would argue the reason it's not happening in the US is because of weak demand, not supply. If you have the sort of "mobile only" demand as seen in WeChat (Facebook is better positioned in that regard than Google), you'd see content providers switch to whatever suits them overnight.