Though you noted this has been already mentioned as not possible, I think they should actually refresh/overhaul old worlds despite the complications. If this means some restructuring or in a sense remaking—I do not think saying it is not possible should be the final answer.
Here is why.
If there is even the slightest possibility of a Create-a-World option (even if it is only for Makers/Marketplace). It would have to be user friendly enough to be able to edit existing worlds (since Makers are in essence using existing content as a template). And even if it does not become available to Makers/Players, I think that eventually there may be enough draw to make similar Store Worlds like in Sims 3. They were highly popular even just being Base Game + the Assets made specifically for the world in question.
So at a Base Game level all worlds might need to be refreshed enough to be feature friendly (such as swimming, kayaking and maybe bathing, etc.), and that maybe like Sims 3 spawners from other packs would auto populate. This could mean even insects and butterflies could be added if you have the applicable packs.
Also to note even there is a chance at cars/vehicles/houseboats all worlds would need an update/refresh, I am sure, so maybe for certain content, all worlds might require an at a foundation level overhaul anyways.
I don't doubt it could be a tangled mess for old worlds to be perhaps basically redone, but if game changing features/gameplay was introduced—players would not like that only the new world and future worlds would only allow that gameplay/feature. Imagine houseboats added and you couldn't add them to any old world, what would be the point of even buy that pack? Same idea for cars.
