> * Data portability guarantees: Vendors must spell out how data gets migrated out, and all formats must be either open or (at minimum) fully documented.

This is not practical for data of any size. Prod migrations to a new database take months or even years if you want things to go smoothly. In a crisis you can do it in weeks but it can be really ugly, That applies even when moving between the same version of open source database, because there's a lot of variation between the cloud services themselves.

The best solution is to have the data in your own environment to begin with and just unplug. It's possible with bring-your-own-cloud management combined with open source.

My company operates a BYOC data product which means I have an economic interest in this approach. On the other hand I've seen it work, so I know it's possible.