This forum contains threads to discuss teams themselves. Anything not technical about the cars, including restructuring, performances etc belongs here.
Because it's to expensive. To be fair, even keeping two lines open should be pretty expensive even if you count the cost of the old tooling being spread over more units.
With the climate controlled dyno rooms, engines attached to mock chassis and real gearboxes, pressurised airboxes, simulated G-forces running through race weekend simluations, I'm sure they can test new developments on the 2017 engine much much faster than the slow turnaround of a race weekend, without leaving the factory.
Only Honda seem to have problems with bringing new engine developments to the track - the rest seem to have confidence and ability to thoroughly test new developments in the factory and then putting them straight into the race.