There would be no point coding anything for the old engine. Sure art and map layout along with conceptual designers and writers could continue as usual, but anything that had to do with actually implementing those things would have to be done once the upgrade was finalized since there'd either be double work to make it work for the old AND the new which would basically double the workload, or the content would have to be "temporary" , only present in the old version and possible ported to the new at a later date.
Technically you COULD ask them to bloat up the labor or half the content release.. but that's not really what they're asking in the OP. They're asking who'd be willing to full stop pause new content so we could get the upgrade faster. This isn't something SE proposed or even has mentioned, it's a whatif.
The company didn't say any of this, this is player speculation. SE, AFAIK, has only said that they're unable to grow the team to make things go faster because of a lack of skilled and compatible talent available. Take that as you may. However I also know that throwing more people on a project doesn't always make it go faster, in fact a lot of times it bogs down workflow and makes it worse.
See brooks law: https://en.wikipedia.org/wiki/Brooks%27s_law
This was because it was understood nearly everything done for 1.0 would be outright trashed and didn't need to be ported (with the exception of some titles and achievements). You can't expect the same solution when things that are made for the old engine would be required to be later ported to the new. Basically any content released would exponentially increase workload.