Well, since it's a Friday afternoon, and I was probably going to waste half my time anyway.....
I checked the all time ASGCA list, and I talked with Ron Whitten.
A lot of guys slowed down, but very few principals just quit. They either retired or ran into health problems, same as any other profession. If they had several courses under their belt, they kept the shingle up. Now, since the 1980's, there were many associates, picked up by firms in the 1980's and 90's, who got laid off, went on their own, managed to snag 1-3 design contracts in the boom years, and then faded away from view (in golf, anyway)
If trained as landscape architects, transferring to engineering, architecture, land planning, or landscape architecture firms has been pretty common, as has becoming park district landscape architects and park planners.
Of course, the snarky answer is "Tour Pros." Only a few founded their own ongoing businesses. Others would get a consulting or appearance fee contract, working with a guy like me, and got letterhead and maybe even an office, but work rarely rolled in, and when the lease was up, got right back out of the business. Or, they were early adopters of the home office mentality.