I'm not advocating for any restrictions to resting.
The current pre-patch system allows every player to rest as often as they deem necessary.
A minority of players, with the current system, only rested when they were out of resources and in places where it made sense to rest.
The majority of folks instead chose easy mode and rested after every combat and then complained that combat was too easy.
By tying any resting restrictions to difficulty you are making players aware that resting after each combat is easy mode and if they are looking for challenge they will need to rest less often.
Y'know this brings up an interesting point. If, as Alodar posits here, the way that EA was configured meant that the majority of players were having an unsatisfying play experience, then that's a clear sign that the design was bad. Just a thought that occured to me. As far as the combination of long rests and difficulty settings, firstly it's worth remembering that this is just a first draft (or second depending on how you want to count) and we're likely gonna see at least one or two more permutations before Larian settles on something final. Secondly, as far as this version of the rest system goes, I think keeping it as its own separate difficulty setting is a better approach. Less because of difficulty itself, more because I can see people not enjoying this sort of resource management, and wanting to play a particular difficulty beyond easy without having to deal with it.