This is a good example of why a tabletop iMac just won't work. The "touching plane" is probably a foot off the surface of the table/desk upon which the computer sits. Which means if your current desk is at a good and proper height for your chair, you're either going to have to sit a foot higher or lower your desk a foot to keep that optimum reach. Doing either of these means your desk is now a foot too low. If you have a pad of paper there, or a cup of coffee, anything- it's going to be a nightmare to keep getting it.
This may work if you built a special table for it and used it standing up, like a kiosk. But then it would only really be useful for goofing off briefly, and not doing anything serious. Who is going to stand for several hours continuously to use the computer?
Touchscreen iMac to me really just sounds like spec-lust and not a practical reality.