Finished reading the manga, and it feels like it was the only logical ending. I didn't really expect for anything special to be at the top and all while they were going up the levels I thought to myself, what's even the point of going up to the top? The fact that while they were going up they ended up destroying the path to go down the levels already felt like there was no coming back from where they were going. The nail in the coffin was them losing their vehicle. Anyway it felt like a much better ending to the story than where the anime ended.