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.
Choosing a stablecoin is the wisest option for a company like Uber. Stablecoins like USDT or USDC have the same value as the US Dollar. This will not confuse people when they first try to pay with stablecoins. This is different from Bitcoin where the transaction fees are high, the confirmation time is longer, and the value is difficult to determine due to its volatility. I personally would rather pay $10 with USDT than pay 0.0001 with Bitcoin. Yes, I think Uber knows about Bitcoin too. But if they are interested, they will make Bitcoin an investment instrument, not a means of paying for their services.