How important is it to flush out dexcool? Why does it matter if you use any other coolant, why cant you just use dexcool and keep it that way?

A shop told me around 500 dollars for a LIM gasket and coolant elbows job, does that seem good?

Aaannd the supercharger oil. How can i tell if it needs changing or not? And how important is it to get changed? What happens if you never change it EVER!? (Not that i will.)