2035?
>The new Mac App Store certificate was using the current, strong SHA-2 algorithm. However, some apps were running receipt validation code using very old versions of OpenSSL that don’t support SHA-2.
I sure hope it's not a SHA-1 expiring in 2035...
You imply that Apple is going to keep using the cert until 2035. I can assure you they won't.
The nice thing about root certs is the private key is only needed to sign intermediate certs and CRLs. This means they can be kept offline in a secure location, and only accessed once every few months or so to sign new intermediate certs or CRLs. The actual crypto typically happens in a special locked-down piece of hardware, so that the actual private key never touches the memory or disk of the computer being used.
The whole process is called a "key ceremony" and follows strict procedures, with technical staff and outside auditors watching every step.
As far as I know, no CA has ever had its root key compromised. It would require physically breaking into a secure facility, or factoring of public key. Considering there's much lower-hanging fruit for anyone attempting to attack PKI, I'm not too concerned with a 20 year root cert lifetime. (Like you say, they could just remove it from future versions of OSX before then!)