The large size 650c track bike market may be extremely limited.
However, there still is some market for 650c kid's bikes, although perhaps they don't move very quickly.
The 26" MTB market is still here to stay for quite some time, including kid's bikes, and the 650b MTB market may well be growing.

I do find it frustrating to locate high quality puncture resistant 24" (520) and 650c tires. Probably not a big market for tire manufacturers, how many tires can a kid really wear out? And nobody likes to spend money on kids bikes. On the other hand, kids can have more problems with flats than adults. One can still buy Continental Gatorskin 650c wire bead tires. With the internet market, I don't see why Continental wouldn't continue to support that tire.

Anyway, I'd expect the kid's market to keep the 26" and 650c market around. However, that may also affect your product selection as kid's needs, track needs, and petite road needs may all be different.

As far as a bike that takes multiple rim sizes, the best thing to do might be to go with disc brakes and identical hubs on both wheel sets. Then the change could be fairly quick. Would you be getting a full custom bike?

Another consideration is rider weight. Say you weigh around 100 pounds, then you may well be much less stressful on your tires and wheels than someone who weighs 200 pounds, and may do better with the skinny tires.