As I understand it, the way certificate authorities are removed from the trust store is progressive: they will announce a date after which new certificates from a given CA will no longer be trusted. I think this can be made even more progressive, by limiting the validity period of new certificates that will be trusted. DigiCert will have little recourse other than to let their customers know, and/or start providing certificates issued by another CA that follows the web PKI procedures and remains trusted. (They can still do that, of course, it's just that their own certificates issued by them directly won't be trusted anymore.)

On the flip side, for user impact, it will play out like this: Some bank or other important entity could possibly, for whatever reason, continue using a (presumably expired? unless DigiCert continues issuing anyways; note that most likely, they will not.) DigiCert certificate after the cut off date, which will lead to users receiving errors. Some of them will have HSTS setup, which will lead to an emergency situation where they have to issue a new certificate ASAP, as it will basically halt their business until they do. For places where there is no HSTS, users may be instructed to simply bypass the certificate warning temporarily, and support lines will be absolutely swamped until they actually fix the problem.

The WordPress situation is quite different. You don't have to use WordPress. Users don't even know what the Web PKI is to find an alternative to it, not that there is one or will be one.

Sure some users would keep the CA active out of fear, while everyone paying digicert would also begin to move out of fear.. What portion of the strange sort of sites that couldn't figure out how to get off but could figure out how to renew would bother with paying for a different error message?