I totally agree, wombat. I wish every video camera could be ergonomically designed to shoot video, had xlr inputs, had the switches and outputs of pro cameras, etc. But like you mentioned, people came to dslrs because of the beautiful image they could create compared to anything else available--and for most people they're going to follow the best image no matter what they have to put up with. But even now, while they have started to make pro line video cameras that have those larger sensors, the price point is still a lot bigger--not as bad as before, but there's still a difference in price range... enough to drive people to the cheaper dslr form. (plus dslr shoot photos, which is a huge bonus for most video guys).
At my job, I shoot on a Sony EX3 that has a 1/2 inch sensor, and it's a great workhorse of a professional camera that's used by tons of people. And I love it and it's a great image and it's got all the things pros need--but in some ways it's inferior to the image of my $900 60D--as far as a desirable shallow depth of field goes because of the smaller sensor. And the 60D is about a 10th of the cost.
All that said, I think a lot of people have grown accustomed to the dslr form factor--they've bought rigs already that allow you to handle a dslr more like a pro, shoulder mounted camera and they're willing to make things work. If big sensor vid cameras get cheaper then people will move away from the frankenstein rigs--but until then... not so much. (and even then, unless video cameras start offering dslr quality still images I'm sure many people will stick with dslrs).
