I worked with a US Yanmar dealer in the late 1970s and we stayed good friends down through the years. So I got to see the whole Yanmar/JD transition.
The JD650 through JD1050 models were manual shift tractors very close in features and specifications to the line of lime green (not JD green) tractors that Yanmar produced in the late 1970s.
By 1980, Yanmar had changed most of thier tractor line to have power shift transmissions with integrated power steering. They also changed the paint from lime green to the candy apple red they still use. Yanmar no longer marketed their older manual shift tractors under their own name, but with a few changes they continued to make the older manual transmission tractors now painted JD green for John Deere.
At that time - 1980 - still JD didn't have a compact tractor line at all, and Yanmar was the acknowledged leader in quality compact tractors. Plus Yanmar owned the patent on the bevel gear front axle which made compact 4wd tractors work so well. So JD made a good choice in chosing to sell those older Yanmars - which already had a reputation for being unusually durable tractors.
The partnership ended up in lawsuits about 1990, which JD won. Yanmar had to shut down their USA dealerships and not sell in the US under their own name for a number of years. But they continued to make tractors for JD.
Like JD, Yanmar is a huge international company making lots of different things. I suspect that for both companies their compact tractor line is a tiny percent of their overall manufacturing business.
Hope the history helps,
rScotty