why there are still so many BP-10 around

Well, they sold for $20 in 1940, which is nothing nowadays, but I remember
my Dad buying a used car for $50 in the 1960s. I can imagine some people
saying "Grandpa paid a lot of money for that radio" and keeping it long
after they knew where to get batteries or cared about using it. Same as
TransOceanics, only cheaper.

They take up very little space, and metal tube portables definitely survive
better than all-plastic ones, which break and are subject to warping from
heat, etc.

Just my $0.02.

Phil Nelson