"Goes down" could include things like "we're going to increase your pricing by 10 cents per charge". It's a much stronger negotiating position that Uber is in if they can say "we can switch 100% of our volume to Stripe in a minute by going to admin.uber.com and flipping a single feature flag".
I don't think they auth per ride. I definitely had a "you owe us money" flow after getting food successfully delivered though Uber eats on a deactivated card.
I get push notifications on auth on both my debit and credit cards. Weak evidence, but my hunch based on what I see there is that there's some dollar value where they pre-auth (somewhere around $25, I think).