Sure they can, it's already been done with the original base game and Horizons.
Most of the narrative involved space back then with only a few minor aspects actually taking place on the surface and only accessible by reaching the surface. But that's not the same scope being talked about here. Here we have the majority taking place (theoretically) in environments that can't be reached with content and assets that dont exist currently so 3.8 wont be able to participate in such content even if you could reach it. Since 3.8 will be critical bugfixes only and nothing else. As opposed to the base game that was not in a "critical bugfix only" state back in horizons release period.
I'm not saying fdev can't do it, obviously they can do what they want, but they likely wont put themselves in a situation where they're selling games that do not result in the advertised expected experience and so the scenario that's being created has conflicting goals.
Limiting 3.8 to just critical updates conflicts with the goal of selling a product (horizons) in a way that reflects the actual game experience players will have (and some players will be limited to 3.8). Not limiting it to just critical updates conflicts with the goal of reducing cost and streamlining development since it would just retain the status quo of maintaining multiple disparate branches of the game.
I'm reading this as all narrative after the Azimuth (aka the head alignment saga) conclusion will be focused on 4.0 but will likely also be weighted towards Odyssey content where it seems base 4.0 users may get to witness the events - depending on the scenario, but won't be able to take part in them unless they have Odyssey.
there's really two reasons to retain 3.8 as an option.
1. Ease players into the reality that they'll be using the odyssey engine even if they dont buy odyssey. 3.8 is a short term option.
2. They can't continue to sell horizons and make this change without the option because it alters the minimum spec of horizons and that becomes a legal concern. 3.8 is not an option but a necessity so long as people are buying horizons under the existing advertised info.
There's no other reason why it would still exist if the whole point of merging is to reduce costs and streamline development. It's existence prohibits the removal of all the backend server side work that goes into keeping both ever increasingly incompatible engines working against the same backend data. which was a major talking point for the complexity and problems around the launch of odyssey and would be a primary point of concern for reducing development complexity and costs.