Vocational education by correspondence has a long history. There was a big boom in it a century ago. Popular Mechanics, for 1920: "Learn the automobile trade at home - spare times" - Dyke's Correspondence School of Motoring.
International Correspondence Schools was established in 1890, and they're still in business. For decades, they had ads in Popular Mechanics, Popular Electronics, etc. By 1906 total enrollments reached 900,000. The dropout rates were high; only one in six made it past the first third of the material in a course. Only 2.6% of students who began a course finished it. Udacity had stats like that at times.
"The regular technical school or college aims to educate a man broadly; our aim, on the contrary, is to educate him only along some particular line." - Clarke, "The Correspondence School", 1906
"I'd aspired to give people a profound education--to teach them something substantial, but the data was at odds with this idea." ...
"At the end of the day, the true value proposition of education is employment." - Thrun, 2013
Not much has changed.