Personally, I think it's for the better they remain separate projects for logistical reasons. I also understand why OHM went with CC0 for their license, and it was their (I think correct) choice to make it incompatible with the OSM one.

I know that's vague, but mainly, I think OHM is better off not having to worry about OSM shenanigans, while OSM is better off being able not to have to worry about historic preservation.