Why is it so common to see survival manga devolve to shonen battler shit?
Because you're not reading battle royale
Same shit with all shonen manga.
They always start off creatively, then once the power and fightan reach the peak level, it devolves into power level where they win by brute strength alone.
I bet Gleipnir is the next to go downhill.
Gleipnir isn't shounen, though. Nor is the magazine action-centric, and the author's other works are all romance. I don't see why it would turn into battle-shounen-style stuff, that seems sort of unfounded.