You could replicate a "perfect display" by analytically implementing motion blurring (which is really just a kind of temporal anti-aliasing) in software. This wouldn't let you track moving objects across the screen without blur, but that's a very niche scenario anyway. Where 120hz really helps you is in slashing total latency from user input to the screen. A 60hz screen adds a max 16.667ms of latency, which is plenty enough to be perceived by the user.