In my country, any city that is profitable enough for Uber&co also already have enough buses. When you already have a bus every 5 minutes, adding the capacity of some vans will not change anything.

On a smaller bus line with less frequency than that, it will also not be really profitable for "independent" drivers.

It may be useful as a temporary solution or a local test but a public transport authority (should) have enough data to scale lines or create routes based on real usage.

When public transport are bad, it's rarelly due to the physcal constraints but always because budget is lacking. You aren't going to solve your lack of bus (drivers) by adding more vehicules with less capacity.