There's other ways they can set about doing it. For one, as I've mentioned, there should be massive attrition rate for Ming in places around China. Historically, these places were in either steppes,tropical areas or deserts.There should also be a buff in the national ideas of the Oirats,Yuan,Manchus and Vietnam. Historically, these countries completely massacred the massive Ming armies sent to subdue them.
Which wouldn't matter at all with a manpower pool of 2m. Once those "barricades" are torn down, the world would fall rather easily. Hell, their WE wouldn't even rise a bit from the attrition.