The fantasy world and D&D game mechanics are two completely different things and should never even attempt to be aligned. The mechanics exist for only one and one thing only... to give the players a game to play. They do not in any stretch of the imagination align to any sort of "reality" within the world. And any attempts to justify otherwise is a person just fooling themselves.
9th level spells exist in the world. WISH as a spell exists in the world. As soon as this becomes known and the methodology for learning and becoming proficient at being able to cast 9th level spells can be learned... any sort of world with any sense of logic and reason would have thousands upon thousands of people learning how to do it each and every year. And to suggest otherwise is silly.
In our world we know how to perform surgery on a person and can save hundreds of thousands of lives by the use of surgery. But do we keep the "secrets of medicine" secret and only have a handful of these magical "surgeons" across the entirety of the globe? No. Not at all. So why would we EVER believe that any sort of cogent game world would have like only a half-dozen people who have ever learned to cast 9th level spells? Especially considering in places like the Forgotten Realms there have been societies and even elves that have lived thousands of years. Tens of thousands of years. All of which time, the WISH spell has existed. And we're supposed to believe Faerun is still stuck in this faux-medieval world? Please. It's stupid and it makes no sense.
So why do we go along with it? Because it makes the game fun. Playing knights and dragons and princessess and magic is fun. And thus we put in game rules that allow us to play all these fun things, even though the logical progression of any of this in "the world" makes absolutely no lick of sense.
So no... we don't limit healing spells in D&D in order to have the world "make sense"... we do it purely so that the game has some tension and risk. And no one can convince me otherwise.
