Ehhh. I think it would be doable. I'm not saying that it would be worthwhile, but I don't think that it would be technically not doable.
From an engine standpoint, they already had an open world for exteriors. Doing the same thing in 3d should be viable.
Separate interiors just lets you devote more memory to an interior because it doesn't need to compete with the exterior. It's an optimization, reduces resource consumption so that you can use that elsewhere.
People have gone back and modded Skyrim to make the closed cities open, and have kind of the same issue:
Open Cities Skyrim should not produce a significant change in your frame rates and performance. Due to the liberal use of occlusion planes in the mod, the game will not render anything on the opposite side of the city walls in any given location. So your viewing content will be limited to roughly what you'd see if you were in the closed city worldspaces. The closed city worldspace system was NOT devised by Bethesda to improve frame rates. It was devised to conserve system memory on the XBox 360 and PS3. You're playing on a PC.
If you are either (a) willing to reduce interior complexity/quality or (b) aren't resource-constrained because you're willing to have fat system requirements, you can avoid loading interiors separately.
As to Bethesda's ability to rip out the guts and change things...from a purely-technical standpoint, going from Fallout 4 -- a single-player game on an engine with a lot of legacy weight -- to Fallout 76 -- a multiplayer game -- was a pretty drastic technical change. I would not have wanted to be the one to do that. Decisions about single-player/multi-player permeate internals all over the place.
By comparison, what would they need to technically do to fly to a planet? Having support for some kind of better lazy loading/preloading stuff. I mean, a planet is gonna have to have a texture. Create a billboard for any structures you put on the surface, and then progressively load them as you zoom in. I dunno if the engine does dynamic level-of-detail stuff in the Z plane today (I think yes? When you're on some of the elevated structures in Fallout 4, I think I recall models way down on the ground being lower-detail), but even if not, I doubt that adding that would be that hard.
I think that a better question would be...how much would it add? I mean, there's an immersion factor, but is doing reentries honestly that much fun?
I've liked open-world space trading games, so I was pretty hopeful when I played Elite: Dangerous, but I wound up kind of disappointed. It's pretty, and maybe if you have a VR rig, it's a good example of something that can create an immersive experience around you. But a lot of elements of the game seem to be aimed more at creating a visual experience than there because they're really great for gameplay. Which for me, at least, was neat at first from a novelty standpoint, but kinda didn't buy me much in the long run.
I mean, I assume that most of the interesting stuff going on is probably gonna be on the ground. I guess you could create some kind of reentry game (flying around storms or something?) but I'm not sure how fantastic of a concept it'd be. My question would be more "what would it buy the player in terms of gameplay to have another few minutes spent doing reentry per planet?"
I'd be more interested in how the character/trait system compares to earlier Bethesda games, how the combat mechanics work, how the dialog with other characters works (a sore point for many in Fallout 4), the kinda meat-and-potatoes Bethesda stuff. Bethesda's had configurable "houses" in Fallout 4 and Fallout 76, and while I felt that there was potential there, I never really felt like the game really took advantage of it; I'm curious how the customizable spaceship plays into this. How well do the new procedural elements work with the static stuff? How readily-modded is the game?