Also consider that trump was in office for the last 4 years making both legal and illegal immigration tough. Hopefully this changes under joe.
I agree with you and many of the other comments concerning the shortage but I'd add that I believe that a major cause (pre-Trump) is that South Korea has been a First World democracy since the late 1980's (1988 Olympics??). So, that factor, together with the reduction of U.S. troops, had made it a lot less likely for a working-class gal to hook up with a GI for life in the U.S. for quite some time. The K-gals who came in the late 1990's/early 2000's (now in their late 40's/early 50's) usually came for more personal issues -- a bad divorce, sick parent, etc. and not political/economic. My Korean friend told me a few years ago that there was a bit of a stream of people coming originally from North Korea, via China, showing up in Flushing but as far as I can tell, they are not showing up in the AMPs in NY.
The C-gals are another story given that large portions of China are still Third World. In fact, Lily of Mott Street, told me that La-La worked as some type of pig farming hand in some remote Chinese village before coming here to AMP. (Perhaps this explains her weird vocalizations during the couple of BJ's I got from her??). So, in my experience at least, the age ranges are all over the map -- mid 20's to early 60's, with the younger looking/prettier/slimmer ones working midtown AMPs and the older/chunkier/homelier ones in C-Town and the outer-boroughs. But C-gal workers are literally all over the world. I read an article in 2017 that the number one destination for C-gals is West Africa as there are quite a few nouveau-riche dudes wanting to marry hot Chinese gals.
J-Gals have always been a bit of a rarity in NY on the AMP scene (and frankly in general). However, I have noticed in the last 10 years a few gals who were mixed Korean-Japanese, raised in Japan or ethnically fully Korean gals who were Japanese citizens/legal residents, popping up many in Korean spots from time to time. This makes sense because Koreans in Japan have long been a discriminated against minority there. Still, they may chose to remain in Japan given that commercial BJ's are legal in Japan, e.g., Blowjob Bars.
As far as other East Asians, back when Julie's NYC was still a thing, there was a young Mongolian gal who worked there known as Keeala, (sp.?) but she was advertised as Korean. I remember during my first session with her, she didn't seem to understand any of my Korean, which I thought was curious. We started to have a bit of a personal relationship and while walking outside she received a call and started speaking in a language that was DEFINITELY not Korean. After she finished her call, I asked what language it was, she had me guess and I told her that it sounded "sort of like Russian" and she told me that it was Mongolian and she also told me that all the Mongolians in NY knew each other and hence when she introduced me to her fellow Mongols, she told them that we met in a bar that she bartended in. The Mongolians look very much like Koreans and as I understand it, the Mongolians and Koreans are close genetic cousins.
I've never met any Vietnamese, Laotians or other SE Asians in the AMP's and only the occasional Thai. I think that the Vietnamese, etc. are more of a West Coast/California thing.