No-one much will stick with W10 on touch devices for long, so anyone wanting to develop a touch app will want a W8 version out there. W10 apps are more at home on the desktop. Hopefully that means we won't get fragmentation issues but if we do MS will need to address such with their current OSs (those still in feature support timeframe like W8.1).
You can set up a dual boot system. I am thinking that way for my desktop but it won't help for low storage touch devices.
Clearly, W10 is the desktop strand of the Windows ecosystem and W8 is the touch strand currently. The only sensible approach is to run both in the same ecosystem until MS provide a viable touch strand upgrade. I don't advise you downgrade any touch device to W7 as that's even worse on touch than W10 (if you can believe it).
Winboy, many people do want to upgrade but want to wait until W10 is as functional as W8. Looking at the current state, a year may well not be enough time for MS to climb that mountain.