I just put a used Japanese engine in the Acura Legend, and researched all the ins and outs of why the engines are here. Emission laws in large parts of Japan are TIGHT!!! In some prefectures they so actively discourage auto ownership that you can not buy a car without providing a letter from the police declaring that you OWN an off-street private stall or garage in which to park it at night. All cars go through increasingly expensive annual inspections after they are one year old - $300-$1500 were the ranges I found when I read info sites on the web.
There is no physical limit on the mileage or ownership that I could find, but on average (smaller country, shorter trips) the cars were 5 years old and 45,000 miles by the time they were judged by the owners to be ready for economic retirement. At that time they could find a foriegn guest worker to sell it to for a song, or, depending on prefecture, pay, give, or sell it to a salvage company for export. As it is cheaper to ship engines than entire cars (more fit onto the ship), most of the metal goes back into the next generation of Japanese cars, and most of the engines ship elsewhere (also saving them the environmental headache of separating the oil and antifreeze from the engine during recycling).
All the above was taken from about 20 web sites, ranging from "We import engine" companies to "So you want to work in Japan" guides from different countries. There may be inaccuracies or perpetuation of legends, but it seems to be the general picture.
Anyway, the Legend has a 'new' V6, which feels more powerful than the original when it was new! The tech says that for some reason the Japanese engines feel stronger than even a freshly rebuilt US engine?!?) $2400 parts and labor. They routinely replace the timing chain, water pump, seals, and gaskets before installing the engine, and as our old engine had just had a brand new distributer installed, the swapped that as well. Looks and runs great, one of the few 5 speed coupes out there, but needs about $1200 in AC work, so I think we will "economically retire" it this summer, or drive it with the windows down.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E