To be serious momentarily, I think at this point they may as well offer a free update so they don't have to bother about differing versions.Those still playing will probably purchase something from the shop in gratitude, or dormant customers who return may do the same if they feel well treated.
While that might be a straightforward thing to do, it's not really a good business model.
It
might be viable for Horizons, given that Horizons makes fundamental changes to the game's architecture, but it's not going to work for more peripheral expansions and it also puts pressure on FDev to continuously come up with expansions which
do make fundamental changes to the game in order to entice customers.
In simple terms, if FDev create, say, a guild-based expansion or a trading-based expansion, or possibly even space-legs, I'm not going to
buy it if I can assume it'll be incorporated into the game for free a couple of years after launch.
In that scenario, the only expansions I
would buy would be ones that dramatically alter the core-gameplay which, as I said, would mean FDev would have to keep coming up with those types of expansion to keep generating revenue.
Given that ED has a projected lifespan of another 3 or 4 years, it's honestly doubtful whether FDev have the time or inclination to create
one more big expansion before time runs out.
Seems more likely, and more pragmatic, that FDev will simply release a variety of specialised paid-expansions and continue to update the core-gameplay for free.
Honestly, the only fundamental expansion to the game that I can think of that's worth having is "atmo' landings" (including all the stuff that should include, such as flora, fauna, seas, cities etc) and, frankly, unless they're already a long way into that, I don't think they have time to build it before ED gets canned.