AGPL also enforces code and changes to be distributed to the users if they directly consume your database over the network. This would be the case, for example, of a database-as-a-service.
Put it simply: you use an AGPL database, internally, even for your SaaS: you're fine. You either modify RethinkDB and ship it as a product or you provide a RethinkDB or RethinkDB-derived database-as-a-service, then you have to also provide source code to your users.
Theoretically we can stop doing that in the future and keep changing the protocol, but that would alienate all of our users. That would be an insane decision, and we'll never ever do it.