it's conceptual: transient means transitory, something that doesn't last long. both tiles are now persistent; what I'm proposing is removing the concept, what you're proposing is deprecating one way of persist.
plone.app.drafts is not needed; we don't use it, we use versioning with plone.app.iterate.
I don't agree neither that persistent tiles have failed, we have been using them successfully for over 5 years.
I agree that versioning is a problem, but I think pushing versioning and all other features (like multi-language support) into tiles is also conceptually wrong. and is not because it can't be done, but because is complex to implement and maintain.
in other words, it add a lot of entropy and we don't have enough resources to keep it working.