You could just throw all your items into the void if you died, that makes it more hardcore ;p. That’s virtually the same as it being able to toggle it.
However I don’t think anyone would if they didn’t have to, just like Claire argued
Turning keepinventory off would be viable if the end were to reset every once in a while. (Yes I know there are enderman farms, but they are made in an hour or 2 nowadays anyway, and knowing the end will be reset one could salvage any valuables.)
Edit: except this wouldn’t help with ‘lag deaths’ of course