If I have that "key" to broadcast the "upgrading" message(the core developer has the key), I think too many clients will be upgraded. I told them to send the private key to me. Then bitcoin is destroyed. It that simple?
There is no such backdoor. What you are describing is an iPhone* or forced Windows Update.
Not everything in technology works like consumer grade devices that have been designed both to protect you from yourself and to allow third parties to take over your devices if you are trying to do something that they don't wan't you to do with them.
*even the infamous Apple didn't (couldn't? I wouldn't swear) remotely wipe the Blockchain.info app when they pulled it from their App Store
And there is a "key" indeed(Satoshi, Gavin, Theymos...have the key). If I am wrong, please correct it for me.