There was a clear unambiguous date agreed upon at the first ecumenical council
At the time they agreed, that agreement produced a clear, unambiguous date. That date now has ambiguity, and these people want to unify the churches. My impression is that you object to changing the date. I object also, but I see some value in unification. That will not happen if our position is that Eastern churches must take our date. I do not consider it reasonable to divorce the date from the lunar calendar. If we want to unify, I think the best option is a "minor" modification to the method from the first ecumenical council.
I may eventually decide that you are right, but I think we should give the date some consideration before choosing between tradition and a unification that would not compromise any key doctrine.
If I recall correctly, the date for celebrating Easter was adjusted at some point in order to celebrate Holy Week during a full moon.