When saying it can be run on old systems and supposedly speed them up - Speed isn't the only thing, other considerations have to be software on said old system.
With no way to do an update install without it effectively being a destructive update, e.g. no retention of the installed software. The user will have to reinstall that if it would even run on win8, will it run in compatibility mode, does the standard win8 even have compatibility mode (without have to have purchased a certain win8 version) and if so at what OS level.
So there is the possibility that some old software may not run on win8, are they to then purchase it again or another that does work on win8, these are the other expenses that I have mentioned before, it isn't just the cost of the win8 OS (reasonable or otherwise).