Well, resistive touch-screens and tablets with that kind of resolution would've been crazy expensive at that time, and probably the hardware as well, so like the PS3, it would probably come in with a big price tag, but i think it would sell twice what the Wii U is doing now due it it not fully abandoning the 'hardcore' scene and reeling in some casuals due to the never-seen touchscreen (apart form the DS), but it probably would've lost even more sales than the Wii due to touchscreen phones coming into play, which unlike with the Wii, would've directly competed with the Wii U for casuals. So i would say probably 45-55m sales lifetime would be the ballpark area.
Edit: It would also have decreased sales due to its pricetag as well.