You can dispose of unique units, why can’t I do it? Have you already figured out what kind of compensation the Mongols will receive? We have already decided on Rus, Rus will receive the Tsar Cannon (The Greatest Bombardment) 
Mongols have Mangudai, which is the Mongol version of the Horse Archer. I’m not saying that you can’t dispose of unique units. I was pointing out a logical fallacy.
Issuing a unit to everyone except Rus and Mongols, do you think everything is ok with logic?
The Rus can still keep the Horse Archer after it has become a shared unit. This happens to be the case with the Imam in which that the Abbasid Dynasty still has access to it after it became available to the Malians and Ottomans.
Don’t worry, the Ottomans will still be able to keep the Great Bombard. How did it happen to the imam 
Now please remember that the Great Bombard isn’t based on the Tsar Cannon. It’s based on the Dardanelles Gun. The Tsar Cannon looks different from the Great Bombard. Also, here’s the list of the largest cannons by caliber. Also, the Tsar Cannon was invented barely within the AoE4 timeframe. So, it might be a bit anachronistic to have it appear in AoE4 and as well as that it might also make the game unbalanced considering the power level of the Tsar Cannon.