The pilot can "know whether he's close to stall without anything more than an airspeed indicator" only if he's not in a turn (see accelerated stall). While you

The pilot can "know whether he's close to stall without anything more than an airspeed indicator" only if he's not in a turn (see accelerated stall). While you are right in theory - if you knew the load factor and indicated airspeed (IAS) you could determine AoA - how would you measure IAS in practice? Normally a pitot tube would give you incorrect readings at non-zero angles of attack, and that's why airplanes use an input from an AoA sensor for correction. Which brings you back to where you started - an AoA sensor. You are second-guessing tens (hundreds?) of thousands of engineers who thought about these problems for more than a hundred years.