FONV was no expansion.
It was a full fledged game the same size as F3 just using the same engine as F3.
Because it plays to both companies strengths and specializations.
Bethesda has a proven track record of creating amazing huge open world games that make full use of current hardware.
Obsidian has a proven track record of creating amazing background settings, NPCs, and creating even better stories using them.
Bethesda will do the F4 DLC and refine and expand their work and then can focus on the next Elder Scrolls while still getting a cash flow from all the work they did to develop the Fallout 4 world.
More money coming in means they have more resources and won't have to rush the Elder Scrolls development.
Especially if they are going to be developing a new game engine for Elder Scrolls 6.
Obsidian can do what they do best and focus on the storytelling using a proven engine that they already have experience with.
They can build on the game assets already created and expand on the already refined game play incorporating the best elements from the modding community.
I really can't see any down sides for Bethesda.
Considering how many years between releases it is not like a another FONV for F4 will dilute their brand.
FONV had almost 10 times the sales of F3 in the first month and boosted sales for F3 just like F4 has boosted sales for both F3 and FONV.
Not only would it bring in more money, but it would keep the fan base engaged and happy.
Maybe they can do a one month release schedule for F5 
Of course if Obsidian has a full schedule and too many commitments, then it might not be possible.
But it is a proven formula to make everyone happy.
So I know they are going to seriously look at it.