I'm assuming that if Larian implements a system like this, then they'll explicitly tell you about the importance of resource management. If people were intentionally attempting to manage resources, then they'd try to do harder encounters when they were full and try to get through easy encounters without spending many resources.

Also, again, this is just a suggestion of a single way to implement a long-rest mechanic. It would obviously require further fine-tuning by Larian. Larian would, for example, reduce the number of supremely hard encounters where you're expected to use all your resources.