Taking Lyft to be a single-purpose undiversified company, isn't the opposite true? I'm no economist nor MBA, but aside from cashflow wouldn't a smaller company be more resilient to conditions due to lower resource demands?

every point of marketshare they get is a zero-sum game against Uber

Is this...true? First, because there's more than two players in the industry, and second because there can be more demand than cars among all of them.