but does anybody think ridership would change if the fleet was, say, all white?
This, right here, hits at the key part of the argument, I think.
I think that the obsession with branding is very much a product of the times we are living in, with the influencers and shortened attention spans and all that. From 1921 until the introduction of the CLRV livery, the TTC used a crimson and cream livery which was hardly original, it feels like a ton of operators used those colours at some point or other including British Rail in the form of their "blood and custard" coaching stock livery, and no one cared. British Rail hardly had a unified corporate identity, either - many locomotives wore a variety of paint schemes throughout their lives, be they steam or diesel. Wikipedia has a not extremely exhaustive selection of liveries that they experimented with over the years, and of course, it goes without saying that the whole fleet wasn't repainted every time a new idea came up, so the fleet could be extremely varied in colours at times. I guess back then, people focused on real issues.
For a more recent example, decades following the fall of communism, Prague has had a multi coloured fleet according to the type of vehicle, with a corporate livery, as such, being introduced only in the last couple of years. It didn't seem to slow ridership up in any appreciable way. I shudder to think that there are people out there who decide whether to ride transit based on the colours the fleet is painted in - I really hope that's just a strawman that the influencers among us have concocted out of thin air!
T3 high floor:
T3 low floor:
Skoda 14 T:
Skoda 15 T (old livery):
Skoda 15 T (new livery):
Culminating in the new corporate livery, introduced in 2020.
All photos are mine except for the last one, with credit going to Daniel Pánek on Flickr.
I have never been convinced of the need for a new GO livery, and the fact that they're pushing to repaint everything now whether it is in need of repainting or not feels like an extremely poor use of funds to me.