I am going to push back on this for two reasons.
1. It was common back then for players to have multiple characters. So having one character killed, or otherwise removed from play for a significant period of time, did not necessarily remove the player from the game.
Yeah, I addressed this in another post. It is a fair point, but means that the design point we are talking about isn't independent - it has other supporting design points, and the thing must be evaluated as a structure.
2. Further, fun can't be measured on a single axis. To give a real life example, when I was much younger, I was spending time with my relatives. My uncle was sitting out on the porch. I asked him what he was doing, and he said, "Watching the grass grow." At that age, I didn't understand it. I couldn't imagine someone finding "fun" in just doing nothing. And yet, he found that to be the best use of his time.
I understand that there are different forms of fun. However, I think this is a bit blithe.
I like having folks over to dinner. I also sometimes actually like doing yardwork. What you're suggesting is that, when folks
expect to be sitting down to dinner with friends, that It is a
good plan for the dinner party to randomly shove someone out of the house with a rake to bag up leaves alone.
Your uncle
chose to go sit outside. The player of Wograff the Barbarian didn't
choose to get paralyzed and have to sit on his thumbs for an hour or more until the combat was over.
I would actually posit that because more people find "fun" in constant engagement, the actual play of a lot of OD&D and AD&D table was less lethal than the RAW (and grognards) would have you believe.
I got into D&D with 1e. I don't generally have to listen to grognards to tell me how lethal the game was or wasn't, 'cuz I was there at the time. The play I recall wasn't as lethal as all that. However, the play I recall did have those long, boring spans for people who suffered one of those long-lasting issues.
Back then, we were young, and didn't have the experience or game design chops to build our own solutions to the issues we had with the game. But game design
itself was young, and inexperienced. I am really not surprised that design has moved on.
To go on with an analogy that I am sure will fall apart eventually but... design has moved on from Model T cars, too.
That doesn't say that there aren't folks who like maintaining and driving Model T cars. But do we really claim that the model T is what we call "good automotive design" today? Accepting that there's always a bit of fashion to be found in design, there are technical aspects to it as well that have more objective measures attached to them. Do we have to say that so long as
someone likes it, it is overall a good design, technical aspects be darned?