That's an interesting point. If L-cancelling were not in SSBM, it would be a totally different game at high levels... unless every aerial attack just had half the lag to begin with. If that were the case, would competitive play really be that different? It'd be technically simpler, and there would be less of a "what if I screw up and get boned" factor, but character strategies would largely remain the same.
Which brings us to the question of "Is the existence of L-cancelling really necessary?" All it does is force players to press an extra button every time they use an aerial attack. One might even say it's just a cheap way to increase the technical difficulty of the game. After all, L-cancelling isn't the same as a skill like short hopping. Once you've learned to short hop, it becomes an option, but not something you'd use in all situations. There are still times where you'd want to full jump instead of short hop--it requires you to think strategically before you can apply it to your game. But there's no reason not to l-cancel once you've learned to do it-- halving lag is never a drawback. So you just do it every single time, almost mindlessly.
Shouldn't technical skill be a tool that creates options, not just force faster button presses?
Just a thought.