Why wouldn't it be as involved as Skyrim? Sounds like it could be more hardcore than Skyrim in a number of ways based on what Bethesda are saying. Just to be clear, I never view streamlining, improving or cleaning up as a sign that something is less hardcore. Lots of people say Fallout 4's character progression was less hardcore than New Vegas and Fallout 3, but I disagree based on what I know now. Fallout 4's character progression system felt much better, especially the way perks were handled. I thought it was more flexible, led to a more specialized character that couldn't as easily just get everything regardless of what path you chose at the start. There's also the factor played by various gear and weapon addons that further boosted character attributes; you could actually go beyond the previous special limits of the older games, building more unique characters.
I just think with them building or upgrading their engine and that that it'll be a true next gen game may mean it won't be as free and open as Skyrim was. I think they'll make it more concise and directed. In Skyrim you could do so many things without doing the main quest. I'd argue the main questline was a distraction from what the real meat and potatoes of Skyrim was, namely, the sheer scope of exploration, side stories and unlockable weapons, gear, trinkets, crafting, dragon song hunting, etc.
This is not to say Starfield won't be great. I think it will be...but I'm not putting my expectations as high as I have for Skyrim. If they pull it off, I mean....hat's off to them, but I don't think it'll approach that kind of depth at least at launch.
With Fallout, I've never fully played through the game. I would do the story mainly and get side tracked with other stuff. I didn't like the base building aspect of Fallout 4 (though, I was a fan of it in 76, go figure) and I just didn't feel a reason to go get everything in Fallout. I can see the line you're trying to draw between Starfield and Fallout, however there is one thing about space exploration that is limiting. You cannot have a continuous open world, if your game spans different planets or maybe even star systems. That may be a small point to make when you're talking about the expanse of space, but the feel of exploration will be different and, I'd argue, will FEEL smaller unless you have multiple planets approaching the size of a Skyrim map filled with things to do like SKyrim has. The Outer Worlds attempted this and was ok...but it was definitely a smaller game