It would try to happen again, because players by nature want the game to be easier. It would be on the designers to push back hard against that change happening.
So it would be better for the game if each action had a 50% of causing perma-death, because harder = better? Perhaps all characters should start off in rags, so they don't have their equipment, which would constantly break, because the harder the game is, the better the game is to play?
Or... is there a point where making the game easier... is better for the game.
You're seriously asking this?
If I (and most players I've known) know nothing can kill my character, things are going to get degenerate in a real hurry.
This isn't "nothing can kill me" it is "this particular fall will not kill me"
Is your game degenerate because your characters know that falling off a horse will not kill their characters, nor will getting punched by a bar maid?
With the bolded, and even though your rationale isn't necessarily wrong, you outright gave them a "We Win" card.
Really? So developing a good plan and executing on it well is no work or effort whatsoever from the players? It is exactly the same as walking into a room and declaring "We Win now!" as loudly as possible?
Look. I know you and me will never see eye to eye on player stuff, because you seem to think all players are forever out to destroy the game and only iron laws of unbending will can prevent them from ruining DnD... but even you have to acknowledge there is a difference between handing out victory and the players having a good plan that you didn't account for.
The way I see it - and run it - if it's good for the goose it's good for the gander; and they know their own tactics and ideas can be turned against them by smart enemies.
And I don't particularly want to get into an arms race where each side crafts ever more complex scenarios accounting for literally everything. It isn't fun for me if the party decides to never separate, in any location, at any time, for any reason, because I might decide to have an assassin group tailing them who will ambush 10 to 1 on a lone party member and slaughter them, just because the party was tailing a member of a bad guy group, waiting until he was alone, and ganking him.
Sure, sure, it could happen. But by all that is good, that level of paranoia would make for the worst possible game. Everything checked for poison constantly, everyone on constant lookout, layers and layers of defenses at all times, just on the off-chance I might decide that this is the time to kill them.
And all pointless, because at any point I can send a dozen ancient dragons, backed up by a ghost army, under the command of an evil god, and wipe them off the face of the campaign anyways if I really, really wanted to. After all, the players would bring overwhelming force to destroy their opposition, and what is good for the goose is good for the gander right? No reason for the god of evil and vengeance to not be constantly scrying to see who is ruining his plans and then crushing them ruthlessly without any mercy.