This thought process (of the UI limiting the player) is a valid point when we are talking about Human vs. AI strength and weaknesses in RTS games. However, when pitting one human player against another, the UI limitation actually
adds strategic depth to the game. Unlike pure strategy games like chess, in Starcraft when you have to move, attack, move, attack, it puts a physical burden on the player. This physical burden to work around the limited UI allows one player to outperform the other on both a strategic and physical playing field, in real time. If both players were freed of their physical shackles, it would become a much more boring display of pure strategy, which eventually would be "solved" and the game would cease to be fun.
If you look at the difference between Starcraft 1 and Starcraft 2, the two most significant UI changes are the ability to select more than 12 units at a time, and the ability to "multi-cast" spells in sequence when more than 1 spellcaster is selected. These two small UI improvements greatly reduce the skill gap between an average player and a "pro" player in Starcraft 2, to such a degree that many Starcraft 2 pro players went back to Starcraft 1, where their fast reflexes give them a much greater advantage against other players due to the limited UI. So an improvement to the UI would help human players beat AI, but it takes a strategic element out of the game when considering human vs. human.