If Larian are worried about players being disapointed about not having Fireball as multiclassed level 5 caster, the issue is information not game balance. Let players understand what they're in for before they commit. Besides, respec gives us a way out of unforseen build issues.
I love exploring Larian's worlds, but am not always a fan of their design choices. I don't really like classless RPGs, I quite like ressource management. I was extatic with the update to Hide and Reactions. I hope against hope the multiclass system doesn't turn into defacto classless.