When migrating over to a new platform it can be a little painful, and we've done our best to give sellers plenty of notice to move over. We cannot automatically port assets as there are too many changes (and the licensing is slightly different).
If a seller doesn't want to move their asset, there's not much we can do. Running the two stores in parallel is also not an option.
I'm afraid it is what it is and we can't go back on our decision.