@Jercov-
Nobility is gone mainly because a democratic system is optimal for controlling the majority of people in a highly productive postindustrial environment. Even when it's democratic in name only.
For a define period of time, nobility was an optimal solution to control a country. Sustainable, with diplomatic benefits, efficient when power is consolidated. The only real fatal flaw I remember is what's called a feudal fragmentation.
When you write an accurate story in a setting like that, it's important to understand that noble kids are not modern zoomers. They grow in a highly competitive and authoritative environment. What's more important, nobility succession system is very rigid. When you're the oldest son and you was chosen by father, you can't just say 'I don't want to play by rules, I want to live a lazy life, make my brother a king instead of me'. Such behavior would be seen as a challenge to a centuries-old system and would be dealt with in the harshest possible way.
