Why would my engine be running perfect one day, and the next barely running only to be diagnosed with burnt valve resulting in engine failure?

My 2005 Volvo XC90 was running fine one day and the next day it was making clunking noises so I took it to the Volvo repair shop right away. I was told that it is likely the balance shaft bearing, but would need to tear down the engine to confirm ($12,000-$15,000)?! The repair shop suggested engine replacement or new vehicle... this car has been well cared for and for something to randomly happen is nonsense (and such expensive problem). I found additional information on the Volvo Forum website with the specific year, model and problem- we are not alone! Sorry.