I don't see it that way. Regardless of the path you take with her as Selune or Shar you are left with a great degree of confidence that she will not stray from that path in the future. If they rewrite her to better fit as being poly then you do not have that same degree of confidence that she would be happy with a monogamous relationship so no, that is not good enough for me. I do no want my character to have a poly relationship with Shadowheart. I do no want her to want a poly relationship while in a relationship with my character. As things stand right now there is always that thought in the back of your mind that she would eventually want to open things up and I hate that. If the game ever gets updated with some supposed definitive edition, DLC, a free patch, or what have you if that is the direction they go with her I will not be buying it and I will never buy another Larian game again. I will accept nothing less than the option to have her say she would be happy with your character, and only your character, for the rest of their lives.
I do agree that it has to be done with serious care. Certainly more than what they've shown the subject thus far, just look at poor Halsin. And therefore I agree that they are better off not doing it, because it would require a lot of resources that would be better spent elsewhere. I'd rather they just add a poly relationship from the start to an origin companion that is added as DLC, or added into their next game. But if they somehow found themselves to have inhumane levels of inspiration to write it properly, I wouldn't deny them the chance if I could (which I can't anyway, obviously). And I think if done properly, it wouldn't retract from her mono story. But that's a very optimistic way of looking at it.
As for definitive editions, they did make those for their previous two titles. I never played D:OS without and I never played D:OS2 with, so I don't know how much those changed. I do know D:OS2 had a very lackluster final act on release, similar to BG3. While D:OS (with the definitive edition) felt more finished, even if it was a smaller game because they had a much smaller team at the time.
For now though, just look at what is actually in her main romance story to help you forget these situations. She wants to start a family with you on a remote farm out of town. Which is exactly what you will do after the epilogue, the game says as much. She wouldn't even be able to open up the relationship there, because there's nobody else around. But that's obvious, because we know she doesn't want to - again, the game says as much. Let that be your memory of the ending.
