Uber and Lyft are only economically viable because they offload their cost of capital — the investment and depreciation on cars and the cost of keeping a driver fed and healthy — onto the drivers, who are only willing to accept such a bad deal because the labor market sucks.
Businesses that can’t cover their costs of capital are not sustainable without some source of subsidy (like the food-stamps for McDonald’s and Walmart workers that we all pay for). If the labor market improves and workers decline to continue to subsidize rideshare companies they’ll need to find some way to get the government or the capital markets to subsidize them or they’ll collapse.