When they start with stable coins I believe that the next step should be to accept Bitcoin because it is the most popular cryptocurrency and Bitcoin holders who wants to spend their coins can patronize Uber for their movements.
The world knows that bitcoin is the most popular cryptocurrency, has the biggest and best community...and I don't think Uber doesn't knows that. But they decided to choose stablecoin as a payment method instead of bitcoin, they must have their own reasons. So I don't think bitcoin will be added later, I don't think bitcoin has a chance.
We should accept the fact that bitcoin is not suitable as a payment method or an alternative currency because of its volatility. Popular payment methods need stability, low fees, and speed, not decentralization and privacy. Bitcoin is more suitable as an asset, an investment.