In the case of such a limited rest, fights such as the bulette or the minotaurs would be much less frequent. Instead, you have to put in simple fights with trash mobs (which are usually much less interesting).
Now how are you going to balance them? Do you assume that the player will be approaching the fight with some resources or maybe he had bad luck in the previous match and will have to rely on cantrips?
If you assume that a player has no more resources then fights can be trivial.
If you assume that the player still has some spells left, the fight may be too difficult.
Now we are adding difficulty levels on which we need to balance it.
Too easy or too difficult fights are an easy way to get bored.
In terms of design, it is a nightmare. You have to consider possible group compositions and a few other factors.
In the game, you are not able to choose next fight depending on how the player is currently doing.
This is part of the problem with adapting an attrition-based game which requires human management to a game managed by an algorithm. At the moment, Larian is using an infinite free long rest system, which removes the attrition part of the attrition based game, and that also has balance implications.
@Rhobar121 I agree that those are potential problems. Incorporating a fatigue (or other long-rest limit) system would
require more work for the design team (not necessarily a bad thing if it results in a better game).
However, most of this game can be done in different orderings. Larian doesn't have to exactly tune every encounter. If the player doesn't have enough resources to fight the Minotaurs, then they can go another path to find easier enemiese:
1.) jumping to another region of the underdark, bypassing the minotaurs
2.) going back to the overworld and exploring the Hag's lair, Waukeem's Rest, the Inn with those Paladins, the Creche, the cursed route to the Tower, or more.
Something your points don't fix is the current balance of short-vs-long rest classes. IF Larian wants every fight to be taken at max or near-max strength after a long rest, then they should also
adjust the classes to strengthen short-rest classes and/or weaken long-rest classes. Given the options of Larian either implementing a long rest limit or making further changes to the classes, I'd prefer the former.