The question of whether letting players police themselves is a solution to the short-vs-long rest issue, and its consequences the imbalance-between-class and the overall-difficulty issues, is a classical question of game design : should designers let the players balance the game themselves ? My answer would depend on the specific context, but in general (and in the specific case of BG3), it's a clear no.
I would be tempted to say that the short-vs-long rest mechanism is clearly and plainly badly designed, but the truth is more probably that it is simply not designed. Yet. And hopefully that will change.
First, let me get 2 somewhat obvious things out of the way.
1) How often you long rest affects the over difficulty. Because more long rests mean more resources to use in fights. This is important because DnD/BG3 is of players-vs-game type (as opposed to players-vs-players). So the challenge level is determined by the rules, not who you play against. And challenge is linked to satisfaction when winning. Most games of that type let the players to choose a difficulty level, but each difficulty level is a well-defined set of rules.
2) How often you use short rest and long rest affects the balance between classes. Because some classes are designed to recover resources at short rest and some at long rest. This is important because DnD/BG3 is a cooperative game. Players on the table rarely want to feel as if the class they picked is just there as a secondary role or foil, while someone is playing the story's lead character or superhero. That is also why most advice for Game Masters include giving every player/character a chance to shine. (All this balance is to be taken at first-order optimal play, when everyone makes somewhat reasonable choices, and that's it. There are too many variables, so min-maxing power-gamers will optimise the shit out of the rules and find crazy combos, and some classes will be better for soloing the game. But that's not the bulk of the game.)
How does DnD handle short-vs-long rest ? Well, the referee Game Master says when the group can and cannot take a long rest, and under what conditions. That ruling is usually given sense in-game. For example, since time passes when you rest (in a the wild, or a dungeon), you have risks of being attacked. Alternatively, since you need space to set up your tent, you can't long rest in a narrow corridor or a slippery and uneven cave floor. But deep down in the mechanisms, the GM is the rules of the game, ensuring that a good experience is had.
How does BG3 handles short-vs-long rest ? Well, it doesn't. I mean, it caps the number of short rests you can take before a long rest (now it's 2 instead of 1, yeah, awesome !), but without any limit to long rests, this is meaningless. It's as if there's no GM, of the GM says "up to you, have a long rest whenever you want".
Let me replace the question of "short-vs-long rest in BG3" by a more clearly numeric one. How many HP's should characters have in GenericTitleRPG ? Let's say that, currently, you feel that your favourite class, the Fighter, should have 100 HP, but the devs were lazy and programmed it to be 255 HP or 999 HP. Whenever your character takes 100 damage, you consider yourself dead and you reload. Maybe I want more challenge and I reload when I lose 90 HP. Maybe someone wants a much easier game and considers 150 HP to be a full life. This is players policing themselves. This has two issues.
A) It's cumbersome. I mean, no, it's just super-bad design. We should be able to enter our max-HP in the game and the reloads take place whenever we reach the threshold. That's exactly what difficulty settings do.
B) It's letting players trying to find where the game is sweet. Indeed, most often, there are some values for the parameters (and other rule-things) that make the game experience nicer. There isn't necessarily a unique set of parameters, and a unique sweet experience (also, players have different tastes). But it's the designers' job to isolate one intended experience and set up the parameters accordingly. Or give us the list of sets of parameters, each associated with a nice experience. Each of these sets of parameters could typically be a difficulty setting.
Now, imagine the devs dropping players in a sand desert and saying "there's an oasis somewhere in there, the water is perfectly fresh, the palm trees perfectly green, really, an amazing experience" but not telling the players where that oasis is. Or imagine a curry playce that doesn't merely let you choose how much chili chutney you want on the side of your curry, it lets you access all the ingredients in the kitchen, make your own curry and cook it as long as you want, while telling you they cook the best currys in town and charging you the same price as any other curry place. To me, that's what letting players make the rules themselves feel.
Note : obviously, my example focused on difficulty, but you can easily adapt it for class balance. If a Fighter starts with 10 HP (without modifiers), how much should a Mage have : 4, 6, 8, 10, 12, 20 ? Just pick as you see fit !
