I really like your text but I just want to offer different view on the guards-are-not-competent paradox of games but on steroids.
Among the adventurers there is a sea of dead people trying to be something. But forgotten and killed by the most pathetic of monsters.
Why not take a cosy job guarding city just taking care of what really needs taken care of?
You have a job, home and you get live in end of the day.
Gigantic rats in basement of the local bar? Why risk your life when you can pay for some smucks to risk their life.
Their is underground cult? Tell the barman to pass around rumors and hope someone else wants investigate.
Instead stand around waving your fancy weapons and trinkets bought from nearby magic shop (probably just general store where heroes dumps their trash).