It doesnt help if that data is being accessed all the time by applications. You just have to break into one application in order to exfil the data or to get the decryption method along with the encrypted data.

'Encryption at rest' only works for data that is not actively used, like backups or if a physical storage device is stolen.

A better additional safeguard is to have quotas and alarms in place for data access. Is data being accessed sequentially in a application environment where data is usually accessed randomly? Is data access bound to individual credentials and do indivudals access more data than usual?

I think, there is actually potential for new database products or addons, which can reduce the impact of breaches in the vicinity of these 'core databases'.