To be sure, Blockstreamers only now are coming to realize that an increase of the effective block size limit to 4 MB will have the unexpected and unwelcome consequence of increasing the effective block size limit to 4 MB.

 So Luke has already proposed to keep the 1 MB limit for the total block size, including the segregated part.

But they should not worry, since the space savings will only occur if the clients start issuing transactions in the new SW format.  Which will not happen right away, if SW is deployed by soft fork.  And even after the clients have upgraded, the use of SW will be optional.  Will there be incentives for the clients to use it?