@rrolo1
As I understand it, mana capacity is mostly determined by the amount of time you spend training it. Whether you're a demon or a human, there's no magic bullet that allows an individual to increase their mana faster than their peers. The concept of a young upstart that overtakes his older peers simply doesn't exist. To put it simply, the amount of mana that one has doesn't lie in regards to how long they have trained their magic and so it is a fairly stable way of determining one's status. When considering that demons train their magic nearly their entire lives, their mana is essentially a representation of their age and that does mean that the position is essentially static. It is unknown whether demons die of natural causes, but it doesn't appear to be the case. At the very least, they an have extremely long lifespan.
Demons have shown that they are willing to suppress their mana to ambush others. They simply don't suppress their mana continuously because there is no value in doing that. The only reason Frieren does so is specifically because she hunts demons and she wants them to underestimate her. There is no reason for a demon to want to be underestimated by another demon since they don't prey on nor challenge each other and the growth of their mana is essentially just an indication of their age. A demon's position in society also has little value in itself and they don't appear to covet higher positions. They collaborate to prey on humans and decide who leads them based on mana.