Bethesda did not prevent or oppose people importing Vvardenfell into Oblivion, they opposed and prevented people from redistributing Morrowind game files and data. If someone were to recreate Cyrodiil themselves or create a program that copies the files from an Oblivion installation it would work. There have been Oblivion mods that allow the user to copy the Vvardenfell data from a Morrowind installation for use in Oblivion. This was acceptable since Morrowind's game files and data was not being redistributed in the process.
Actually, Morroblivion DOES use MW assets which was the real problem. By "assets" I mean models and textures. Even with the retextures out there, Morroblivion still uses Morrowind models for buildings etc. Bethesda doesn't own the rights to give people permission to use Morrowind assets in other games because they didn't create them, they paid someone else to.
If a converter for Oblivion => Skyrim came out it would become a banned topic on this board instantly almost certainly.
Honestly, these converters always svck and deliver sub-par results, I don't know why anyone would want to straight convert game content. It would be far more inspired to create your own heightmap and then use the editor to put towns etc in place. You're going to have to recreate all the quests anyway if it's anything like the Morroblivion converter, so why even bother converting the Oblivion quests? Are there seriously people here boring enough to want to replay all of Cyrodiil's quests in the Skyrim engine when they COULD write all new quests that focus on the state of the empire during the time of Skyrim?