Here's why I don't think that downloading TV shows off the Interwebs is a real threat to broadcasters:
I can get better quality picture and sound for FREE over the air with a simple antenna. Plus, I can get THAT faster than I can download it (often before it is even available for download).
Downloading has some advantages, I guess, but I have a DVR to capture off the air, which gives me most of them anyway.
And yes, for quality one can download the hi-res versions of a show, but a single episode of House is about as big as every episode of Important Things with Demetri Martin combined.
Just an observation.
(Also, if you ARE a downloader, and you have a fast enough connection, check out Hulu. Some of their streams are faster and higher-quality than your average cap, and the commercials are shorter than over-the-air)