It is impossible to introduce a hard rest limit, which means that the balance will fall out of the window anyway (there is a way to do it, but Larian won't introduce it because players will eat them alive)
Even Owlcat has practically given up on any attempt to limit resting (this psuedo restriction in WOTR is no restriction if you can find items that remove corruption at every step.)
The current system is just more convenient but it has no effect on balance anyway as long as rest isnt heavy-restricted (which will not happen)
I agree on two counts : spell preparation can't be restricted to long rests at camp because that just encourages spamming long rests and Larian won't heavily restrict long rests. However, freeform spell preparation is only one factor in the imbalance between classes.
Druids, Clerics and Wizards can cast a prepared spell, unprepare it and keep the benefits. This works with, among other spells, Mage Armor, Barkskin, Protection from Good and Evil, Flame Blade... This gives the preparation spellcasters more available spells than what their spell limit would suggest, which is a distinct advantage over Warlocks and Sorcerers.
Wizards can learn any spell from any class. Given 150 gold and a few scrolls (Bless, Healing Word, Guiding Bolt), Gale becomes better than a Cleric.
Though, in DnD, ritual casting lets Wizards cast spells that haven't been prepared, it only works with selected spells and it takes 10 minutes. In BG3, preparing, casting and unpreparing a spell can be done with any spell and in one six-second turn. Also, ritual casting is a feat which should be available to anyone with enough Intellignece or Wisdom but doesn't exist in game. Again, Wizards are buffed while other classes are nerfed.
Even without fundamentally changing the resting system, some of these issues could and should be adressed.
