I mean, it was called the sweet spot because it was the range of levels where the game worked best, or, really, worked at all.
There's no way functionality is a bad thing to design in.
But, markets are funny, and human nature is even funnier. If for whatever reason, your customers would feelzbad if high level play actually worked or classes were actually balanced, well, you have to cater to it... You're a business, businesses cater to customers to make profits, within the limits of the law. If catering to one set of customers makes more money than catering to another, and catering to both is not an option, you pick the more lucrative market.
...it'd be interesting to dig into
why (some) D&Ders hate balance, tho... is it just being accustomed to it's absence for so many years of the game's history? Is it the prospect of losing hard-won system mastery? Is it the 'revenge of the nerds' theory that gamers identify with bookish wizards and nurse school-days grudges against the 'jock' fighter?
....