A most interesting theory you put across, tarmpropp.
Acknowledged as it is to be the most loved team in the Peninsula, the Old Lady remains second to Torino in terms of popularity in the city of Turin.
However a vast majority of those Fiat workers (Fiat and Juventus were founded the same year, or nearly I believe) do claim to be Juventus fans.
This may be partly due to the fact that the Agnelli own Juventus, but also because many of these workers are southern italians who emigrated to the North looking for a job.
Indeed Juventus is all the more popular in the southern regions of Italy, as a game they played in Palermo some time ago in front of a 50 000 attendance testifies. This is why they did envisage to move there at some point, since they were guaranteed to see the exuberant sicilian fans fill the stadium every weekend, unlike their detached (or Torino-supporting) pre-alpine counterparts.