I am pulling this out of my head but most broadband is asymmetrical. Putting more bandwidth to downloading than uploading making the connect more efficient. Unless one running a server one dose need much bandwidth for uploading. One could have a symmetrical connection but it will cost you on bandwidth so instead of down/up of 20Mbs / 5Mbs one is left with 14.5Mbs in each direction.
I don't think cable's DOCSIS bandwidth works that way. There are separate upload and download streams (frequencies/channels) and raising the upload doesn't mean you have to give up download. At some point what you say could be true. Cable plants have ~1Ghz of usable spectrum and until the combined download and upload consume all of that spectrum, changing either doesn't mean the other has to lose.
The coming DOCSIS spec has some lofty claims of possible 10Gbps over coax with a 2Gpbs return path.