Like others have said, I don’t think 5e is the game for trying to “realistically” model faustian bargains and fey contracts. Character build and mechanics are primarily how players express themselves and have agency. Messing with that is against the spirit and intention of the game.
Maybe you misspoke. Maybe I misunderstand. As written this is a very narrow view of the game. It relegates the game to mechanical drudgery, stripping entire letters largely out of the RPG. And it appears to contrast, starkly if I may add, with the rest of your post.
For many tables, even some very famous ones, mechanics aren't the primary way players express themselves. Backstories wouldn't be so popular if mechanics told so much of the tale. We can look at the r/LFG subreddit for hints as to this folly here.
Casual glances show repeating trends in the way people frame and express the games they wish to play. One such frame is "roleplay heavy." Many such threads emphasis character. Character in a vague sense, in a person within a fantasy world sense. In a fundamental part of the game sense.
We can look to the popular youtube personalities related to 5e. They speak volumes about characters, roleplay, and, as one says, the "talky talk."
If mechanics were the only legitimate form of agency, the only form of expression, these frames, these word choices, these ideas, wouldn't be nigh-universal in the community around 5e.
I don't think we can so easily dismiss the storytelling aspects of a storytelling game as sub-servant to the mechanics. Instead we should see the game as an interplay between those aspects and the mechanics.