Um, not sure what your trying to say here. The TT rules for resting are largely irrelevent, and always have been. You rest as much as you want and as often as you want. If this requires going somewhere safe, and you choose to do it, then you do it.
The cost to the player is exactly what the DM chooses. The standard trade-off used to be the chance of "wandering monsters" while asleep. But all this achieves is to ensure that resting occurs even more frequently to ensure you still have spells for attacks while resting.
One of the early sets of modules D1-D3 that introduced the Drow, actually denied arcane casters ANY spell recovery when in the Underdark. So guess what; when the arcane users ran out of spells ( or had used a certain portion ), the whole party trooped back to the surface to rest.
And yes, you have reasonably analysed where Larian sit as DM. They know there is no point in trying to force any particular resting cadence ( which would be wildly unpopular ), so they design on the assumption you will rest as needed. Typically, I find I get through 2-3 fights before needing a long rest, due to the choices I take. But if I needed ( or wanted ) to long rest after every fight, that's exactly what I would do.
The only thing I would disagree with is that there is any real or implied relationship between "balance" and the TT resting rules. Nothing in the resting rules forces any particular behaviour upon the player, so any intent to use the resting rules as a "tool" for class balance woud be an astonishly poor design choice. I suppose that is possible, but if so, I don't see that it should be Larian's responsibility to fix it.
My point is that Larian, if they want to allow unlimited resting and/or not implement a day/night cycle, should more fully commit to this.
They have achieved arguably the most important thing, which is balancing encounters assuming players will rest as often as possible.
But they need to do a lot more work on
1) the camp cutscenes that only occur during long rests and can be missed if you don't long rest frequently enough.
-Option A: More strongly encourage long resting. Companions could say "let's return to camp; I have something to talk about" instead of just saying "I'm tired." Alternatively, some sort of light penalty should apply when companions start saying they're tired. Maybe a -1 to skill checks...not enough to force a rest, but enough to more strongly encourage it
-Option B: Untie these long rest conversations from long rests, and allow them while traveling or while short resting
2) Consumables and Food could be reduced, so that it becomes more necessary to long rest frequently. This, again, encourages players to long rest without explicitly forcing it
3.) If we are expected to long rest frequently, then honestly it wouldn't be bad to make some class changes. Give warlocks more spell slots or wizards less. Maybe adjust abilities that recharge per short rest (e.g., Action Surge) to have more uses. This would have a similar effect as allowing wizards to use all their slots in 1 encounter.
And in PnP you can't always just rest whenever you want; as you say this is up to the DM who sometimes declares that you cannot long rest. Or you can, but the DM imposes some penalty, like enemy reinforcements arrive or you have to waste time in which brings you closer to some deadline. Larian as the DM is allowing unlimited long resting but rarely imposes corresponding penalty.
