Since you mention that an alternative is a concrete wrapper type with both stream and continuation as properties, then would it be the case then that you could use init instead of a static creation method with that alternative?
Since platform owners and not the Swift Evolution process control what is back-deployed, I would be very hesitant to compromise on the proposed design of a feature on the basis of what can be back-deployed. It would set a very poor precedent to contort the design of permanent additions to the standard library on the basis of transitional bring-up matters that are not even within the full control of the Swift project.