I'm not a piano tech/tuner, but I noticed that you've not received much of a response. So I'd thought I'd give you my opinion.
First: I have no experience with Boston pianos, so I have no opinion on them.
Bias alert: I actually own a Baldwin L, though not a new one. Mine was built in 1966. I bought it used (obviously) just last year, and the instrument was not well-maintained. Here's the work I've had done to it: New white key tops (the originals were chipped a bit); new key bushings; new hammers (Abel); complete regulation; voicing (string leveling, hammer prep, etc.); and tuning of course. What I now have is an instrument whose sound is very, very nice indeed.
I don't think all
that work would have been required if the piano had been well-maintained. I think a well-maintained Baldwin will hold up quite well over a long time. I think others in this forum who are far more expert than I have the same opinion regarding longevity.
Regarding the Shigeru Kawai: My experience is limited to an extensive "test run" session last summer at a dealer who had three of them: the 6'6", the 7'0" and the 7'6". Of these three I thought only the 7'6" was worth consideration. It was a very nice instrument, but not worth the $58,000 asking price. (Nor worth the $42,000 price the salesman was offering.) The others just did not impress me. For me, the Baldwin L has the better sound (compared to the Shigeru 6'6"). But others will feel just the opposite. I have no idea about the Shigeru's longevity.
Hope this may have helped some.