It seems that everyone thinks that keeping the base game updated is somehow hard for FD to do. If the likely updates are what people are expecting (Legs and Atmos, based on being on the road map from the start) it's easy to lock people out of that content.
The problem isn't keeping the base-game updated.
The problem is ensuring that owners of the base-game don't accidentally get screwed over.
We've already had minor examples of this where, for example, there's been CGs that are supposed to be open to
everybody but which require things that can only be found on planet surfaces and (unless I'm mistaken) non-Horizons players can still meet up with Thargoids but they
don't have the same access to AX anf Guardian tech that Horizons players do.
And, of course, at it's most basic, a base-game player (with no access to engineers) can still get attacked by fully engineered opponents.
The simplest solution is to ensure that all expansions are "exclusive" (in the sense that they're all completely separate from each other) but that means each expansion is likely to be rather narrow in scope, which is a shame because it means FDev will never be able to create an iteration of the game that showcases
all the expanded content.
For example, we already have Horizons.
Let's say FDev (by some miracle) manage to add atmo' landings
and space-legs.
If they want to avoid forcing players to purchase all 3 expansions, they won't be able to create any space-legs content that takes place on planet surfaces and they won't be able to create any surface content that requires space-legs.
That means, you're never going to get the "full experience" of dropping out of orbit, flying to a city and getting out of your spaceship and walking around to complete some objective.
Course, with 3 or 4 years left before FDev can ED, they're probably just thinking in terms of 3 or 4 minor paid-expansions and that's your lot.