Two spaces grew out of a typographic convention of using a 1.5-width space that was favored by typesetters for proportional typespaces because typewriters didn't have half-width spaces represented on their keyboards. The actually "correct" approach would be 1.5-character width spaces, because even in proportional typefaces a little extra space serves as a useful visual cue that aids in quicker text scanning by eye.
The modern convention of using a single space is the result of journalistic publishers' desire for economy of printing paper. It costs more -- either money for extra pages or characters that won't fit on a page -- to have two (or even 1.5) spaces between sentences. For that reason, a new convention for non-personal correspondence arose, not out of "correctness" or readability concerns, but out of the miserliness of accountants.
As for the lack of studies, that's because it's pretty difficult to come up with a meaningful set of criteria that can be (relatively) easily measured in such a study. Worse, the people with both the resources and interest necessary to fund such studies are for the most part not interested in finding out their cost-saving measures make it harder to read their publications. People I know who read a lot -- who enjoy reading -- including myself all agree, though: having more than a single (proportional or otherwise) character width of space between sentences helps with making it easier to read quickly without having to backtrack and without missing things. In fact, if anything proportional typefaces makes the problem worse, because the spacing between sentences tends to end up smaller than it would otherwise be.
For all his annoying certainty that people who are certain of their disagreement with him are annoyingly wrong, Farhad Manjoo is pretty laughably lacking in the proud correctness he claims.