I will be in Paris from Sept 24 to Oct 4. Someone just informed me that Paris fashion week falls inside that period. I am a museum nerd and wont be trying to spot celebrities, but I will be taking public transit a lot. Will the crowds be worse?
The fashion week crowd does not take public transport. Surface traffic is indeed worse, so buses in the central areas can be slow, but it has no effect on Metro or trains. (On the cost of hotel rooms, yes !)
Busses in the center could come to a standstill, drivers will open the doors because walking is faster. The metro will be unaffected.
By accident, I was in Paris one year during fashion week. And yes, surface traffic was awful (it took me two days to figure out the reason) but in specific areas near where shows were being held and in the luxe hotel parts of the 8th. I would never have known about Fashion Week because I don't hang out in the "right" places but I had an appointment in the 8th arrondissement and made the mistake of taking a taxi.