I keep getting confused on the actual number. I read both 860 and 870MHz - depending on which site I'm getting information from. 1GHz is certainly an option, but, again based on my understanding, implementing it has its issues. For example, for regular cable anyway, there are lots of areas in the country which aren't even running at 750 yet (as you correctly point out, cable systems currently don't run at 870, and even if they do, they need to use some of that space for things like internet, voice, guide data, and vod - Verizon doesn't have that issue) - they're running at much lower frequency ranges. Second, rate of power loss increases with frequency. The higher the frequency, the faster the drop-off rate for the transmission of the signal. For cable that's a problem because they need to provide a mechanism for boosting the signal more often.
For FiOS that's not an issue since we all power our TV signal from our houses. And that ONT runs plenty hot. I have no doubt that Verizon could easily migrate to a 1GHz system if they wanted. But to be honest, I don't think their original plans ever included what they're doing now. I think they created a system with enough space to get them to full IP - they must have figured that they had enough space to get them there. Then DirecTV launched a new satellite, and with it a few dozen more HD channels, and the race was on. Now Verizon has to upgrade their system (get rid of analogs, and upgrade to use the full 870 MHz) to add all those HD channels. I think that will allow them to delay the migration to full IP for some time. When this transition is done, they'll have way more space than they'll need for some time to come - and more room than any other provider out there - for now. Their longer term solution has always been, however, to migrate to full IP. I just don't see them having a need to migrate to 1GHz service following this upgrade - their next big upgrade will be to IP.