Using cloud as a glorified hypervisor isn't really getting the benefits. To be cost effective you really do need to be using the value-added or layered features, and those are proprietary.
I disagree (obviously). There are huge advantages to using the data stores from your cloud provider, but you can pick Postgres or similar options that avoid lock-in. If you use DynamoDB or Spanner, you can work to avoid non-portable features. Many other services like SNS can be replicated with software like NATS.
That said, switching from one cloud to another will never be trivial, so it may be worth using some proprietary services up front if it gets you to market faster. The key thing is to have your eyes open about the choices you make, to ensure you have the ability to switch (or, perhaps more importantly, threaten to switch) in the future if you need to.
Sure, but if you restrict yourself to lowest-common-denominator features so you can migrate between Spanner and CosmosDB then you are paying for features you aren't using and are at a competitive disadvantage to those that can leverage them.
That said, switching from one cloud to another will never be trivial, so it may be worth using some proprietary services up front if it gets you to market faster. The key thing is to have your eyes open about the choices you make, to ensure you have the ability to switch (or, perhaps more importantly, threaten to switch) in the future if you need to.