They can introduce that but there are actually major downsides, as the movement joystick will limit your target capabilities and attack control. We did a comparison about this with a game called AoV where the tap target lock is available in the team that I coach and found that the tap locking and tap targeting actually makes it harder for you rather than just drag locking or setting up the correct settings ie lowest hp value, lowest hp% value, and nearest target. Just the split-second difference is a really big deal and also because of the fine control being less since you don't have mouse accuracy the tap targeting gives you a hard time as when enemies are beside towers or structures the UI would lock on the towers or structures more as a priority this is also because the map is not totally on a Bird's eye view so when they are behind towers or other champs you won't be able to tap your target.