I have 50-60fps on "empty" planet by doing horizon stuff, basically. Just being landed. I had 100+fps in horizon.Concourses and settlements will be inaccessible without the DLC. And performance in space is actually quite good by now.
Space I lost FPS to, since I'm not capped at 120fps, and sometimes drop in the 100+ fps. While it's good, and not important, it means the performance is worse, and someone who have 60-ish fps in space in Horizon is going to suffer.
Performance took a big hit, even for Horizon content. While it's playable for me, it wouldn't for someone who have a less capable computer with 60-100fps in horizon.
Then you have literal game breaking bugs and various balance bugs in Odyssey. If it was merged, I couldn't do CG anymore, simple as that. I also had issue in CG giving engineering item to a contact, I couldn't give them in bulk, and one by one was a big no-no.