If you have your own cert, wouldn't it be best to ignore expiration (and decide based on risk whether or not to implement revocation)? Unless you are saying to add the cert to the device/computer's list of trusted certs, which doesn't seem necessary to me.

All of Google's pinned certs are certainly evidence that this is a reasonable approach (I haven't looked at their expirations), though now it's kind of weird since it can't be MitM'd even to debug -- no one knows what is being shipped off to Google by Chrome anymore without reverse engineering binaries.

Issuing inordinately long-lived certificates might not be the best security decision. To put it another way, "infinity" is not the appropriate lifetime for a 4096-bit cert.