Having only one ISP locally is typically either caused by market size or local regulation. Comcast does offer a national backbone and sells transit, cdn and other similar services through a subsidiary called Comcast Technology Solutions. The backbone at a national level has definitely received upgrades and they are advertising multiple terabits of capacity. Of course upgrading the transit network does not help if your local link is saturated, which is a problem in some areas.

If there is truly demand for unmetered internet someone will step in and fill the void. Some ISPs also use total transfer as a way to discriminate between business internet and residential internet.