Well i didn't mean to say that's the purpose of the time limit. There's only two conditions to create a loop: die, or run out of time.
You don't often die, so the game is designed around having limited resources: time, ammo, etc. As to create a need to use the loop mechanic.
It might just be my incompetence but i find it especially hard to design around this mechanic. Not a good start for my 1st ever puzzle game, and my 2nd game, well ever.
But i get what you mean - it's not worth it to design the game for multiple difficulties at once. It makes the game boring for players that actually care about the game.