MobileMe replaced .Mac, which provided crucial functionality like email. There were many fewer people affected, but the situation is similar.
To quibble: Mobile Me was a thorough replacement for iSync.
At a higher price.
But iCloud (which he "blessed" and introduced at stage) replaced Mobile Me and killed iDisk (wiping a couple million web pages off the Internet), keychain-syncing, MobileMe gallery, iWork.com, ...