Bolded portion: I think it got overlooked, and that's my own fault, but I did already mention that Frontier has stated that they will be contacting base game owners some time after Beyond to discuss options. That leads me to believe that the number isn't that large, but it's good on Frontier to reach out to that group anyway, and try to take care of them. Due to this, Horizons being the only version of the game going into the next paid DLC is almost certain. I honestly hope that Frontier just upgrades those folks for free, at this point. No reason to charge them money, just send them an email with the code, and good instructions (including Steam-specific instructions) for how to upgrade properly.
Riôt
I think Frontier have played themselves into a corner and what you describe is probably one of the only moves they can make, yes.
otoh they could remove the ability for non-horizons owners to continue to play until they upgrade, which would work for a larger but inactive non-horizons playerbase.