Cheating degrades the quality of education at any level. Cheaters are trying to get accreditation when they shouldn't. While Cheating is all over in education (we had a couple of cases at my school), it is obviously going to be more prominent in a place like Coursera, where cheating is easier and looking for cheaters is harder.
One thing they could do is create an algorithm that checks answers much like what they do for journals, but again that's an extra load they "shouldn't" have on their employees.
Not in the least! You can argue that it degrades the prestige or something like that, but the education remains the same.
The university has the onus to hire credible teachers, and to maintain a high reputation. You're paying for the standards they apply. Some universities aren't worth their weight and some are. The information that they have is out there. This has not changed. Textbooks have been arround for a while. Additionally professional teachers have the ability to communicate the material affectively.
Online classes can't guarantee a level of understanding of the material. Additionally online classes will never be able to guarantee that the person who signed up for the class is the actual one that gets the credit.
It wouldn't work at Coursera's scale and for a price point they can afford.
A: "Oh hey I know how to program Scala"
B: "Ok program Scala..."
A: "Well I only got the cert, I don't actually know."
B: "Sorry I guess you're not qualified then. Why don't you learn?"
Yeah that sounds like a great interview. How long do you let them go, half an hour, a day? Do you think you can adequately judge someones abilities like this?