Wow the ending is interesting and people seemed to think the manga got axed, im pretty sure it didn't. Because if it did, there won't be a "sequel".
So the question is why did it end like this?
Well in my opinion the sequel where it have a different mc, is to view the world in a different perspective. Because you have to remember that the underground world is huge and the first mc only come across 2 cities, so the sequel mc is going through other cities.
Remember that this series isnt a shonen where this one mc become op through hard work and conquer all the cities in the underground, this is a seinen series.
You just have to treat the ending as watching a movie, this whole manga is 1 movie of a universe, so no wonder the ending isn't an "ending" because it's still has more to offer.
So you kinda have to read the sequel to see more, and it's interesting to view more of the f*ed up stuff in the different cities of the underground. And im betting the ending will have all the mcs fighting together to destroy the underground district.