It's easier to manage 1 database instead of 1000s
Same logic allies to compute boxes, see "pets vs cattle" from 15-20 years ago.
The whole point of a database is to contain the state - as a pet - so the rest of your application can be stateless - as cattle.
To really get cattle database systems, you need a self-managing cluster architecture that puts things on autopilot like Neon where you've got >=2 copies of each row and can tolerate losing any single box without unavailability.
But restoring a small DB from a fresh backup, if things go really wrong, is faster, and does not affect other customers.
I completely agree wrt having a hot spare / cluster with transparent failover and management.