Gamebryo is based upon NetImmerse anyway... And the Creation Engine is based upon Gamebryo, you would be surprised at the level of inter-compatability between all three engines.
I think however... It would make more sense to port Fallout 3 to the Creation Engine and go from there.
I don't have much doubt about them bringing those games forward to the Creation Engine if they ever get the green light. After Skyrim, that would be the most logical choice for stability and versatility with mods. It would probably still be a nightmare but at least they have experience with that process.
The real question is not why Bethesda is now supporting Nintendo. What matters is, besides the upcoming Doom and WF 3, can the engine(s) for the upcoming Bethesda games allow for scaling the games to Switch? If not, despite their good will, we won't see support for much longer.
In sum, we probably/most likely won't see ES VI, Starfield and Rage 2.
If Starfield is still using the Creation Engine from Fallout 4 then I think that really hurts the chances of it appearing on the Switch. Not unless the've greatly enhanced the scaling capabilities well beyond what we've seen with Fallout 4 and 76.
The next Elder Scrolls ... who knows. It's years away and is likely being made with next generation in mind. However, people seem to think it will have a new framework, so perhaps it will appear on the successor to the Switch, whatever that might be.