(Gudetamago's Girls' Last Tour manga spoilers)
Agreed. To add to this: The way I see it, at some point the girls realized that their journey to the highest level was a one-way trip. For them it might have been when their Kettenkrad broke down and it became obvious how much having a vehicle had helped them survive for so long, but looking back we could notice how pathways tended to collapse after them and how they were repeatedly told that few resources were left in the entire city. There was nothing to go back to even if they found a way to go back down. We were told indirectly that Kanazawa and Ishii hadn't survived, why would Chito and Yuuri fare any better? And with them progressively abandoning or using everything they were carrying, their trek to the top felt increasingly final with each chapter. By the time Chito started burning her books I don't think they were even expecting to find anything up there anymore.
I thought their last words about figuring out what to do next when they woke up were similar to when Chito burned her diary and Yuuri told her "You'll be fine, you have a great memory"—empty reassurances and not quite wanting to accept death yet even though they knew it was inevitable.
Short version: I think of it as a straightforward depressing ending with the silver lining of "Oh well, we made the most we could out of this life". Painful, but in a good way.