Most Populous Modern-Day Countries in 1800 AD

There may have been as many as 20 million people there except for the disease brought by Europeans in the 1500s.

What I mean is that Bangladesh was at this time the seventh-most populous country in the world. Nowadays, it's eighth.

That just means other countries are growing faster

Cahokia collapsed well before european arrival, true, but there were still fairly large-scale Mississippian towns and villages up through the De Soto expedition in the mid-1500s as accounts from that expedition describe several settlements with temple mounds and populations in the low thousands. That particular expedition likely marked the final population collapse of large-scale indigenous settlements in the area, as it corresponded with a disease outbreak in the Southeast and in the Mississippi valley, not to mention the looting and killing De Soto did as he marched from town to town. Even so, there is a fairly clear continuation of culture between some regional Mississippian subcultures and modern tribes like the Muskogee, Natchez, and Caddo. There are accounts of the Natchez at least living in walled villages with central temple mounds up into the mid 1700s.