Operations and Support seems to be the smallest chunk of that at 2.3 billion. I am not sure if that's the pool of money drivers are paid out of. It would be better to see the actual cost paid to drivers instead of assuming "a big chunk", as to me it's looks like a smaller chunk of the smallest chunk.
If the executives received no salary they could pay the drivers an extra 0.2% or so. If the executives received no compensation whatsoever they could pay the drivers an extra 1-2%.
[1] https://investor.uber.com/news-events/news/press-release-det...