Back when we played AD&D with henchmen/hirelings, a player getting paralyzed by a ghoul or carrion crawler.... well, we never had a conversation about "this is disruptive to our enjoyment of the game." Probably because combat was significantly faster. And because there were fallback NPCs the player could hop into the role of until their PC was back on their feet. Aaand because we were kids/teens without a mature understanding of game design. Whereas with modern D&D (3e/4e/5e) combat has a longer handling time, so the player feels that "being out of the action" more.
Anyhow, while I agree with the premise of modifying the Stunned condition, I think it's even more important to remember the big picture intent = maintaining player engagement. IOW there are other levers we can manipulate (e.g. making combat extremely fast, or having fallback NPCs/companions, or flashbacks when stunned/taken out of play, or giving every monster with stunned a specific weakness/workaround unique to its manner of stunning), which would get us to the same end goal.
Effect wise, that's true, stunned is rough, but it isn't the most terribly impactful of all the conditions. But you also need to look at how easy they are to remove. For example, compare it to Paralyzed.
Ending Unconscious just needs some magical healing - it's the easiest to remove. It also goes away on its own in 1d4 hours iirc.
Ending Paralyzed requires Lesser Restoration (2nd). It usually lasts until "save ends."
Ending Petrified requires Greater Restoration (5th). It lasts the longest, being permanent until intervention.
Ending Stunned requires Power Word Heal (9th). It lasts either "until end of next turn" or "save ends."