Indeed, it's one thing to keep an old bike running under the "ain't broke don't fix it" rule, and another to bring new wheels into the world in an obsolete size. All of my new wheel builds are 700c, even on ancient hubs such as old Sturmey Archer and Bendix.
On the other hand, a possible use of obsolete sizes is if folks are literally giving the stuff away, then you might be able to assemble a perfectly good if not great bike for next to nothing. That seems to be the case with 26", which hung on for a few years longer than 27" did.