"Can you show that the existence of a skeleton key introduces risk beyond losing the key?" The fact that losing the key is a possibility is risk enough. Once PFS is implemented, the only way (barring crypto attacks) you can break an TLS session secured with it is to have compromised the systems at the time of the communication. A skeleton key now means that there is a possibility of offline decryption with just having a copy of the communication and the skeleton key. This key is handled by humans now, instead of machines and a protocol. That is far more than double the attack surface area.