We're talking about system fonts here, not webfonts, so they can differentiate as much as they like with webfonts. Webfonts would still be available. Although they'd also have to go into the per-site cache to prevent sites from fingerprinting by whether or not you already had the font loaded via some other site.

(Web fonts can be quite space-effective, if the site can serve a trimmed-down version that doesn't have the whole unicode space in them. CSS even has support for breaking the font into pieces so if a bit of unicode does slip through on some page you can still go get "the rest" of the font.)