https://news.ycombinator.com/item?id=37286891
The user 'pessimizer' replied that there was a company OptiFont that seemed to do something just like this, to sell cheap font clones:
http://abfonts.freehostia.com/opti/
http://luc.devroye.org/fonts-27506.html
It still seems so relatively trivial to automate this -- just render every character on a <canvas> at a fixed known font size at super-high resolution and autotrace outlines, and render every character pair to figure out kerning, and the common ligatures. The only major thing missing is hinting, but that's irrelevant for printed material and not very important on retina screens -- and auto-hinting gets you more than halfway there on non-retina. (And then there are all sorts of special font features like nonstandard ligatures, which can either be more carefully replicated, or just ignored.)
I'm not saying it would be a good/ethical thing to do this (I personally want designers to be supported) -- but I am kind of amazed that the market for digital fonts still exists, when in theory you can produce a perfectly legal copy and not pay a thing.