It's not that it's a bad mechanic, it's that it's a dumb one, an unnecessary one. Performing it does something good for your character, not performing it does the opposite. Again, not a bad mechanic since technically it benefits your character, but it is the only technique I can think of that is both positive for the character and not necessary. It is, simply put, superfluous.
There is, again, no reason to not perform this technique, so it is to everybody's benefit (and every character's benefit) to standardize the landing lag among the cast. There's no catch to it, there's no 'gigantic change' that anybody would have to get used to. It's removing something in order to speed up the play (or, rather, keep it at the same pace). It is removing a negative option for all characters that literally nobody chooses. It is always giving the player the best choice for something that they all choose anyway. It's streamlining the experience, which has been one thing Project M has been offering since day one. Keeping poor design choices around just because they've been around do not make those choices good ones. I'm positive that if you ask people if they'd want a "PRESS ME TO NOT LOSE" button that must be pressed every time they attack they'd tell you no.
You could have everybody and their dog agree that being able to halve your landing lag on an input is fantastic, but there is no reason anybody could argue for "would you like to have a choice between having twenty-eight frames of landing lag and fourteen? Or just fourteen?". Absolutely no reason. Seriously, ask yourself that, and try to convince yourself that this is a sound technique that has something serious to offer to this game after years, and years, and years (over a decade) of development. Ask yourself, "would I rather have to choose between landing with 15 frames of lag, or 30, or just land with 15". What would you rather do? Why do you even want the option there to land more slowly? You never hear a commentator point out that "that was such a sweet L-Cancel" because everybody's doing it and is supposed to be doing it. It is not an impressive-to-behold technique, or one that is worth mentioning. It's worth mentioning even more when somebody does not perform this technique than when they do.
There's no alternative or replacement technique. "Land with 25% the effectiveness of an L-Cancel, like somebody suggested". So, what, we end up with choosing between landing as fast as we can, or more slowly than that. Ask yourself a third time now, "would I rather land as quickly as I could, or have the choice to be more punishable". Does it still sound like you'd choose the second option, ever? Out of tradition's sake? You'd hold back the game because that's what you're used to?
On the subject of gutting technical skill. 'Technical skill' refers to, of course, being able to perform a variety of techniques to at least a high degree of success, and being able to apply these techniques in the most applicable of situations. A technique in itself does not present the player with any higher degree of skill until they are of course able to perform it, and beyond that apply it in the manner that allows them to dominate their opponent with no room for them to counter you. Let's look at some common techniques, and the gist of them.
Wavedashing: an unorthodox movement option allowing for surgical spacing of moves, fast fake-out options, and attacks while moving. Technique usefulness varies character-to-character, it not being a universal need-to-know technique. There is some lag associated with the technique, and as distances/speeds vary, certain characters will benefit more strongly than others, and some benefit more strongly in certain situations.
Perfect Shielding: a mechanic through which you block an attack on the frame your shield is activated, akin to a Parry. Allows the character blocking to react instantly, taking no shield-knockback or shield damage. Precise timing is required to master this technique, and the timing is universal through the cast. There is no reason to not do this 100% of the time (though, it would be extremely improbable to do so).
Fast Falling: A movement technique involving a motion on the control stick to speed up your character's decent during any of the types of jumps. Allows for specific hitboxes (and only specific hitboxes) to be active during certain moves, providing certain applications for autocancel windows. Allows for various timing options with attacks, giving the attacker another mixup tool. Speeds and timings vary through the cast, and they are non-universal.
L-Cancelling: A technique that reduces the landing lag of aerial normals by half. Any aerial normal can be L-Cancelled, and is a technique that is strongly encouraged to learn early on so as to speed up your game and leave fewer opportunities for punishment. The timing on the actual cancel is universal throughout the cast, and the benefit is the same (halving landing lag). There is no reason to not perform this technique 100% of the time, and is not difficult to do so.
Now that we're through with that, look at each of those techniques, and tell me what would be lost by removing each of them. The list for removing L-Cancelling is a lot shorter, isn't it? Removing L-Cancelling and standardizing landing lag rates at their L-Cancelled rates removes nothing from the experience since everybody worth mentioning is doing it to begin with.