@MajinSweet
To be honest, I think your perspective of depth is flawed because you give it such a narrow criteria.
In simple terms, depth should typically be viewed as something that adds intricacy to an aspect. Saying that viable options is the only thing adding depth to a competitive game is like saying that symbolism is the only thing adding depth to a book; it is just not as simple as that.
For example, it is a fair statement to say that viable options adds depth to the game, however , parallel to viable options is decision making. Decision making can only exist if viable options exist, but deicision making is ultimately what makes viable options intricate in a competitive environment. Ultimately, both of these things add depth to the game and their interaction adds even more depth. Depth is really just a fancy way of saying complexity.
So I am not going to list all the possible forms of "depth" in a game (I am not smart enough anyway) but I am to go ahead and say that execution difficulty and execution requirements adds depth to the game. Execution serves as the fundamental limitation preventing the player naturally being able to do whatever they are capable of doing within the game. In order to overcome this barrier, they have to practice and learn the most effective ways to learn execution. This may not be "idealistic" depth, but it does serve to make the game more intricate. Furthermore, much like decision making and options interact, execution has its own interaction with these concepts. Oftentimes, execution can give access to better options... but it also can serve to impair the best decision making since your mind has to process another task.
So adding complexity = adding depth and interaction between these things = more depth
With all that said, my other main point is that you cannot criticize L-cancelling on the basis of its lack of decision making. This like trying to fit a round peg in a square hole. You are trying to judge it based on a criteria it was never meant to be in. Understand that L-cancelling is purely an execution mechanic for effectively perform an aerial close to the ground. If you L-cancel, you aren't gaining options, you are not losing them. It is completely intended to have no decision making.
To clarify, consider the action of short hopping where pressing the jump button quickly is required to perform it correctly. The decision making is entirely in the short hop; once you decided to short hop, your are required to press the jump button quickly. You never decided to press the jump button quickly, it is what the game is asking of you.
Similarly, L-cancelling is part of correctly executing an aerial attack while landing during the animation. You are never supposed to decide to L-cancel, your decision was made for you when you decided to aerial while landing; now you are required to L-cancel.
So this was pretty long winded but here are my two main points,
1. Depth is not just viable options. Decision making and execution also add depth to the game and they all interact with each other
2. You can't argue against L-cancelling for not incorporating decision making because it is an execution based mechanic, not an action such as shielding, jumping, dashing, or attacking
With all this said, I do think that the one valid argument against L-cancelling is that it adds seemingly arbitrary difficulty to the game. The question is: do you prefer what it adds to the game over its difficulty?
The trend of games these days is to make things simpler and more accessible, however, with simplicity comes a loss of depth (complexity). Consider the differences between Bioshock and its predecessor System Shock 2. In terms of immersing you into the environment, both are very effective. However, System Shock 2 had far more intricate rpg mechanics that gave players a larger set of options for picking and choosing how they want to play.
So yeah, if L-cancelling was gone, this game would be easier. But there would be a loss of depth also making the game "simpler". Whichever idea you idealize more is personal preference.