WPF went that route by using vector graphics everywhere. It has the nice property that it scales fine, but often you can see small one-pixel jumps of elements, e.g. the thumb in a scrollbar or on sliders. This wouldn't matter at all on a 300-dpi screen but with 96 dpi it's visible and sometimes jarring.

We don't have super-high-resolution displays. A small percentage of users have moderately high resolution "retina" displays and everyone else has low res. One major benefit of the pixel-based 2x design workflow is that it produces good results on old low-res screens that are still in the majority.

To some degree I think you're correct, but until there are only retina displays on the market we need a solution that works well for both. Everyone looks forward to the day when designing a font doesn't mean spending half your time hinting for those pesky ~85ppi displays, but we're off to a pretty slow start. The first real step in my opinion is to bring real vector support to the web (now that it's reasonably well supported across browsers) but that requires designers to adjust their workflow (and preferably own their own retina displays to test with).

We won't have to make fonts for Retina - we already did, they're (usually) designed to be printed which is much higher DPI than a Retina display.. It wasn't until Windows XP when Microsoft said "Let's make fonts that look great on the screen, at low resolution"

Yes, that was my point (except it wasn't just XP– hinting was/is a PITA on any platform). These days we still have to make them look as good as possible on both, but if you're also targeting printed media then you're already set when high PPI displays are the norm.