Serious question - why hasn't the Uber board replaced Kalanick yet? While it is indisputable he successfully brought Uber to where it is now, it doesn't seem like he has the good judgement to be leading Uber at this stage of the company's lifecycle. Wouldn't the best thing for the company at this point be a complete overhaul of leadership?
The new CEO will most likely get some key performance objectives related to current problems as well as revenue (or margin) growth.
If there's a reason to believe TK can commit and be evaluated against the same indicators, the incumbent CEO is a better choice since no time is wasted on onboarding and getting familiar with the business. If he refuses to commit or commits and is then unable to execute, now that stipulates a much more productive board discussion on how the current CEO should step away to let a "more mature" or "more tenured" CEO that can execute against those objectives.
So if any resignation is imminent, expect it in 3-6 months, not now.