I think you should cross-post all or most of that on Talk:Typography Refresh, since not all the designers are actually on Wikitech. A few thoughts of my own...
This doesn't seem like a satisfying leap forward, given the level of disruption.
1). I don't really see how there is or will be actually any serious "level of disruption" for users beyond a temporary shock of an incremental change. Right now what I see internal MediaWiki community drama over changing a longstanding precedent, which is not the same thing. I don't know about you, but I care ultimately what happens for users of MediaWiki/Wikimedia not whether people want to fight about something on a mailing list. If that's the measure of disruptive, then almost everything we do is extremely disruptive.
As you say, it's not a hugely radical chance, and so far overall use of the beta has stayed steady or grown over time. Millions more mobile readers have been happily using almost the exact same font family settings for all our projects for more than a year. (That's the basic consistency we're talking about.) Feedback from most end users on the Talk page has been positive or neutral the base content font family, or has been more concerned with other details (like serif headings).
People here mostly seem to have debated two points: "are violating our free software requirement?" (unequivocal answer: no) and "is this better?"
The answer to that is partially a matter of personal taste, which is why everyone who wants is still free to customize the skin however they like. Typography is one of those elements of design that everyone has a strong opinion about, even if they don't really grasp fundamentals of the domain. The second part of the answer seems to me that the UX team, as the experts in this, needs to do a better job of documenting their research and the feedback so far from end users. That is why I recommended we hold back on pushing the beta feature to Vector for now.
2). I think Jared and others hinted that the most satisfying leap forward would be delivering free/open webfonts to all users. Ideally, it would even be something custom designed for Wikimedia. However, that's just not a realistic bet right now, and we have reason to tread cautiously as Erik said.
3). Last and most important, you listed main body font family, but ignored the other changes that are a part of the Typography Refresh as housed in Extension:VectorBeta. A large part of what makes typography good or bad is not simply the font family you choose. It's how you set it -- the visual context -- that matters just as much. Ultimately arguing about that one line of CSS is not really a discussion about the actual experience on our sites.