Is a leaking valve gasket signs of future issues?

2013 Audi Q5 with +150k miles is in the shop for new spark plugs and a fuel pump replacement. Shop calls to tell me the sparks are covered in oil and carbon build up from a leaky valve gasket that seems to have been there for some time. They suggest repairing ASAP, but warn this could still lead to engine misfires and knocking which would mean end of life for the engine.

1. If the valve gasket is replaced this week, how likely are knocking and misfires to follow?
2. If an engine does start to misfire or knock, can this be repaired or is the engine essentially dead?

Thanks for any insight