I wouldn’t assume it’s exclusive. At the scale of Uber, it may be worth keeping both systems running and split usage between the two to negotiate the best rates.
"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).
Ahh yeah, and also at the international scale. The article also states that Stripe is expanding internationally (Didi Chuxing, Grab), so it could be that Stripe handles Uber's international payments, whereas Braintree handles the U.S. ones.