Marketing would be negligible because Uber has already created that market: "that other app where there is no driver begging for tips like on Uber" would not need much help selling itself (assuming uber would be phasing out drivers incrementally, while a robotic competitor would start 100% driverless).

As for capital costs: I would expect competitors to have a much easier time raising capital for their fleet than Uber, where all new investors have to to share their stake with several billions (!) of pre-selfdriving investment. Not every competitor will be drowned in investment, (there is an infinite number of ways to share off investors) but one or two will be enough to cause serious trouble for uber.