If we wanted to deal with randomness and chaos, we'd be playing with items on.
This gets into the details of what I said. Most people don't actually think about how items actually affect play. My opposition to them is because I've actually sat down and determined that not only can you not adequately prepare for them (in that the preparations that you do cannot possibly mitigate their effects), and the act of preparing for them drastically limits gameplay (in that it comes down to intelligent use of a small subset of tremendously overpowered items, and understanding of traditional mechanics of spacing break down completely once the items start spawning).
You can't quite adapt to star and screen kos, but you can prepare for them, and preparing for them only limits gameplay if you aren't willing to gamble in a small subset of situations.
That's the thing. This doesn't limit or enhance gameplay. It has no real value to the game. It's one thing if dying off the top is delayed compared to other blast zones: people are probably ok with death not being instant there. But there's nothing to gain from having 2 different death animations especially when it's determined randomly.
If there were different death animations based on % or how fast you were flying, or xyz aspect, sure it's fine. If something happens and you lose because of it, THEN you can use the intuitive "should have planned for it" card, like well you could have tried to kill him vertically sooner or if he passed the damage threshold you should have been aware and tried a horizontal KO. But being randomized does it no favors at all. There is no adapt, no "use it to your advantage" since it's random, and it doesn't add anything unique to gameplay since this all takes part after technically dying anyways. You're already hit, already sent past the blast zone, already dead, now flip a coin and see whether you lose the stock sooner or later??
Like you said. It doesn't limit gameplay. What it adds, though, is interesting in how it affects the choices the player makes.
Because there is something you can do about it. If you're so worried it will change the outcome of the match, then you can choose not to trade upward KO moves with your opponent. That's the safe option, and no one would fault you for taking it. But if the odds are in your favor that you'd win the exchange, and no other possible exchange favors you, then taking that exchange is worth taking a look at. That puts a very unique constraint on the decision-making process. That's what the mechanic brings to the table.
To approach the issue from another angle, you're looking at the topic from the perspective of "we want to change this and we can, so why don't we?" I'm saying that's a pretty terrible reason to change something. There have to be self-imposed limitations, otherwise anything goes, and that can be just as disastrous. I'm applying the Turnip Threshold to ott kos, because if we really wanted to do away with randomness on the level of ott kos, turnips would be getting changed as well.