I took a look at some of Lyft's metrics [1,2,3], but would be interested in seeing a more nuanced breakdown of their expenditures. You mention driver signup bonuses are a major expense, and is "The only reason they aren't" profitable today. Is this a fact?
Wouldn't it be far more sustainable to just boost base rate with the money instead to reduce churn? It sounds like a perverse incentive of metrics to reward new drivers instead of total ones.