What has not been explained is how cylinder 8 was scored.
The explanations by the dealer all revolve around carbon build-up in cylinder 8. As far as I know (and that, admittedly is not that much) carbon build-up does not cause scoring in cylinder walls. Engine cylinder scoring is caused by extreme metal to metal contact.
Why pull an engine because of carbon build-up on the valves on cylinder 8?
And why change out the valve springs in other cylinders besides cylinder 8? Why even change valve springs at all? Unless, of course you suspect the springs are bad or, a valve spring actually broke. Now that could lead to scoring in cylinder 8.
In some engines, the valve spring plays a role in keeping the valve from falling into the cylinder. When a valve spring breaks, the valve associated with that spring could possibly drop into the cylinder. As you might guess, things could get a little messy in that cylinder with a valve rattling around in there.
Possible explanation (fairy tale) for why a new engine: Dealer finds lo compression on 8 & carbon build-up on #8 plugs. Looks further, finds broken valve spring with valve gone. (dealer/BMW panic, at this point). Pulls engine and finds remains of the valve and scoring all over cylinder 8 walls. BMW, saving a few bucks, says, lets sleeve the cylinder and put it back in, he'll never know (my cynicism is showing). Puts engine back in but sleeve job doesn't hack it. Result-new engine. End of fairy tale
I'm glad you got a new engine. I think if I were you I would have done the same thing as far as keeping the car goes.