I think we need to standardize on some terms. We're talking about data transmission down a pipe rather than broadcasting EM radiation, so "bandwidth" is a better term than "spectrum" or "frequencies". Each pipe provides a fixed amount of bandwidth, and within each pipe, regardless of the physical technology (coax versus fiber versus beamed microwave [sat]), the bandwidth is divided into channels of a fixed bandwidth each.

Not quite. That data transmission down a pipe is still EM radiation across set frequencies. Those frequencies have a cap depending on the plant being used - usually 750Mhz or 1Ghz for newer or upgraded systems.

And there's the rub: Uncompressed video needs at least 6 megabits/second (or its equivalent 6MHz analog channel) to transmit low-rez (NTSC/PAL) video.

Yep, that's analog for ya.

The pipe has a fixed amount of total bandwidth, and the more you can compress each channel the less bandwidth it consumes and thus the more channels you can pack into the fixed pipe...which is why it is an economic issue. Each type of provider uses a technology that gives it a certain amount of bandwidth, and adding more is expensive. They can essentially create more "product" at zero expense by cranking up the compression, quality be damned. Maintaining quality while adding channels means investing big bucks in new infrastructure.

Not big bucks, huge bucks, and mostly in the plant, not in the fiber/coax end. And then you're still limited to 1Ghz of bandwidth with current technologies. The solution is to scrap your analog signals, but to do that, all of your non-digital customers need a cheap dig-analog converter. That ain't cheap either.

BTW, resolution is another factor that the carriers can play with to wring more revenue from their systems. The broadcaster may transmit 1920x1080p, but nothing prevents the cable company from downsampling to a poorer resolution which requires less bandwidth.

Agreements with the broadcaster can limit it. The public limits it. Witness the 3:1 compression of HD that Comcast got caught with in early summer. The fault was in the compression algorithm - after the outcry, it got better.

Back to bits/second...The fact that each channel has a fixed bandwidth is why breakups and tiling are compression artifacts. The break up case, e.g. an explosion in which one frame is entirely different from the previous frame, instantaneously exceeds the available bandwidth--more bits have to be transmitted in 1/30th of a second than the channel will accommodate--and so you get breakup and pixelization (tiling). The more you choke the channel to wring more channels out of a fixed pipe, the more vulnerable each channel is to compression artifacts.

You're mixing two different things.
The more you compress the more you have compression artifacts.
No matter how much you compress, you're still sending the same amount of data down the pipe.
Breakup, tiling etc are due to RF interference in the plant or in the cable or in the home, or due to overloaded servers pumping out the video. Very little to do with the compression.