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.