One thing (I don't think) has been mentioned is that there are sound technical reasons for trying to consolidate the game around Horizons. Currently FDev have to support two versions (branches) of the game (base and Horizons). This adds a lot of overhead in terms of coding and testing. If you add another branch to support, then it quickly becomes untenable. The code would end up with lots of corner cases, switch statements, branching logic etc. It would be a nightmare to maintain. Much better to merge everyone into the latest branch
Currently FD really, really want everybody move to Horizons. It's a win/win for them - way less technical issues, possibility to say goodbye to DX10, less support costs, and of course money from sales. That's why they are seriously sweetening the deal.
Also this time is bought by abandoning season concept. I assume some of ED developers are working in Season 3 in background (whatever it is hard to speculate), while rest of team keep upgrading core game, base stuff which rest of the game builds upon. I suspect unnamed expansion 3 announced at next Frontier Expo in 2018.
Reason why FD would want Horizons to be new base game is obvious from technical stand point - it uses all shiny tech they have developed for developing new things (like planet surfaces). To move forward they really have to move as much playerbase as they can to EDH.
I expect them to face similar problems in future when let's say they will need to move to DirectX12 due of performance requirements. Although with their own rendering team which seems to have very professional people who deliver results it might be less of an issue. I guess DX11.3 will stay true de facto render for ED for foreseeable future.