. . . I can come up with is that the machine tools (and to some extent, materials), as well as the extent of skilled labor, used to produce watches have changed markedly since then.
To replicate Thomas' watch today in precisely the same way it was made then could cost multiples of what it would take to build within the constraints of current production technology. Even manufacturers like Nomos that cater to the lower range (price-wise) of today's consumer make extensive use of computer assisted design and manufacturing.
My sense is that if it weren't for Grand Seiko, any semblance to mass watch production of the Golden Age would be gone.
Art