Um, I help a bit with organizing Google IO, also in Moscone West, also capped at 5K-ish, also an instant sell-out.

You say “Moscone Center is huge. Apple could blink and triple the size of the event” and I really disagree. Once you get over 5K you move from Moscone West to Moscone N/S, which is a vastly higher level of commitment, financially and logistically. On top of which, Moscone W is a nice light airy space, while N/S are basement shit-holes.

Now, your argument concerning exclusivity is probably valid, but I think the amount of work/money involved when you bust out of Moscone West, coupled with the quality of the attendee experience, are very significant factors too.