That's not an enormous margin, but it's large. Uber is still expanding rapidly in driver count, in geographic range, and in product range. That takes quite a bit of money, especially the geographic range - Uber runs pretty vigorous ad and lobbying campaigns to influence regulators and oppose taxi agencies.
Fasten is a startup whose entire business model is "Uber but better for drivers". They advertise to consumers that they take a smaller cut, and hope to attract more drivers with their better rates. My guess is that Uber will crush or acquire them after dropping prices (or raising driver share) in the relevant markets.
So I think the answer is: Uber could do this, but it would slow their growth. Once they're an entrenched player in most markets, I would expect to see driver share (and possibly rates) rise a bit to maintain supply, but right now they're funneling everything they can into expansion.
(And, I'm sure, panting at the thought of using self-driving cars to turn that 80% into profit.)