Having asked my question here, it occurred to me that I have a good bike resource close to home in Kenilworth, IL - RRB Cycles.
I just want to point out they could have tried to sell me a bike; instead they recommended staying with what I have.
Mr. Boi's judgment is
1) Yeah, losing an inch or more in the legs is pretty common as his customers age.
2) The way I've always ridden keeps me pretty safe from the main injury that I fear. (My feet are flat on the ground when I straddle the bike, even in cycling shoes, so stopping and starting are fine.)
3) It's too good a bike to give up on now - it's a Brit, all Reynolds 531, never raced, although it's gone lots of relatively easy miles in Chicago's North Shore and Southern Wisconsin.
If I start to ride a lot again, I'll probably get a new smaller bike. (I just missed a red 19"/50 cm Super Course on eBay.)
Mr. Boi suggested further seat adjustments, which I will do as soon as I find my 10 or 11 mm wrench.
I got my bike in late Fall, 1981, just after my beloved 54 cm Atala Competezione was stolen. It was sized right - 21" seat tube, 22" top tube. According to Sirotta's system around 1986, the seat tube was less than a cm from what he'd have made for me if I ordered custom, but the top tube was short!
Turin Cycles in Evanston had taken the frame in trade and basically gave me used parts needed to build the bike. I built my wheels. It's got a very unusual set of parts - Avocet/Ofmega, TA, Suntour, Campagnolo, Zeus. Clincher tires.
One very unusual feature: the fixed BB cup is Italian; the other is British. Apparently the previous owner cross-threaded the fixed cup trying to install a Campy BB. Turin re-threaded it. Given the help they were giving me, I didn't ask for the other cup to be rethreaded - after all, it wasn't broken.
It doesn't look great, but it's pretty sound mechanically.
The ride up to Kenilworth was my first ride out of my neighborhood in 5 years - sure felt good.
But my birthday gift desire has become clearer - bike shorts that fit....