For the l-cancel you need to interact with the opponent as well, heck for every action you need to interact with your opponent, if that wasn't the case the game would be pretty pointless.But Powershielding requires that you interact with your opponent from them throwing a move out. Your opponent can interact and cause you to not powershield with mix-ups.
This is depth.
L-Cancelling opens options, but why not just axe it off and cut lag in half. You get the same game at a top level without that extra barrier.
This is not depth, this is a needless barrier that hinders the game.
You throw out an aerial at your opponent then there are 3 different outcomes:
Miss, hit or hit on shield; This changes the timing of l-cancel so you have to anticipate the opponents reaction to get your l-cancel right.
Now the opponent guesses that you approach with an aerial, he could throw out a light shield to mess your timing up so you miss the cancel and the opponent created a punish situation because he cleverly interacted with you.
L-cancel is barrier you're right, some like it, some don't like it, but I wouldn't call it no depth.