This isn't an overnight change, we have 3 years until the 47 day certificates go into effect.

In terms of increasing their capacity by the 10x, having shorter certificate lifetimes means that CAs will have a shorter list of valid but revoked certificates, and also will have way less of valid certificates in the certificate transparency logs. These are checked constantly, so the reduced size means less costs serving this information.

CAs are already charging an arm and a leg for very little work of signing the certificates. Doing domain validation is an automated process, so unless the need is for OV certificates (which doesn't differentiate you anymore in modern browsers), CAs won't need to hire more people for issuing certificates. With Let's Encrypt being a free option that supports ACME, if CAs use this change as a cash grab, they'll probably see clients move away rather than put up with the outrageous costs.