Isarra Yos asked:
Has it ever been specified why mw needs a font stack in the first place?
"mw" may not need a font stack at all, maybe "the site" here is only WMF properties. Steven Walling, could you clarify the goal? As Jon Robson pointed out earlier in this thread, WMF can have settings to only apply the font choice to Wikimedia projects.
When and if the typography update moves out of Beta feature and into the Vector skin, MediaWiki could go out with straight serif headers and sans-serif body, while WMF sites get the carefully-deliberated font stacks.