Enjoyable read and a lot of good points.
From a SaaS perspective, multi-tenancy support is not on the radar for DB developers, yielding duplication effort as every SaaS provider creates yet another SaaS helper service.
Some sort of data API (Behold! The Vapourware!) could realise a lot of these challenges, as well as tackling challenges like tenant aware authorisation, database routing, partition balancing, DB-engine-per-schema options, etc.
There's still a long way to go for the most used component in our SaaS stack.