Well it's getting more and more complicated as the story goes on (like a good story should) the fights (at least the major ones) can no longer be explained by, oh! Here's a super strong RANDOM person that appeared out of nowhere, Let's fight!
And if they WERE explained like that the fights would have less impact as the people the MC(mino) is killing have no background to us so we wouldn't care as much.
My opinion tho, take it with a grain of salt.
It happened in the novel. The minotaur became a side character starting on volume 2 and the new MC is Panzel's son (there'll be a big time skip shortly). Let's wait and see if they'll do this in the manga as well. I honestly hope they just end the manga with the ending of volume 1, which is approaching.
what having him just wait at the bottom of the labyrinth is a horrible decision from a writing standpoint. Have him go up top now and fucking reap havoc if you want an interesting series of progression. Have him like band together other monsters, he surely can't just be written away to wait for the new mc to show up just to defeat him.