>unless I've missed some monetary/power advantage

the power dynamic here is that the CAs have a "too big to fail" inertia, where they can do bad things without consequence because revoking their trust causes too much inconvenience for too many people. shortening expiry timeframes to the point where all their certificates are always going to expire soon anyways reduces the harm that any one CA can do by offering bad certs.

it might be inconvenient for you to switch your systems to accomodate shorter expiries, but it's better to confront that inconvenience up front than for it to be in response to a security incident.