Remember HTC Wildfire 1? My first Android smartphone, BTW. It had this cute optical sensor built into the home button, and it could be used to position a cursor while typing easily just by gently swiping your thumb. Imagine having this kind of optical mouse in a modern 6"+ smartphone. Imagine being able to reach everywhere on the screen with a thumb only, while holding the phone with the same hand. Why doesn't this exist?

Yes, there's Quick Cursor. But it should be a built-in feature, a hardware-based one, with no extra fees.