The question is what happens if these sorts of micro-contract work arrangements are outlawed... is the expectation that these businesses will instead hire these people full time, thus increasing full time employment?

Maybe, although it seems unlikely. These sorts of tasks aren't going to be worth most companies hiring someone to do it full time. Instead, the work simply won't be done and the business will just be a little less efficient and responsive.

The other alternative is that someone would start a specialized service providing each of these types of tasks, and hire full time workers to do the work, and then contract out the services directly.

Is that better for the workers? Maybe for some, they will have full time employment. But will they make more money? Maybe, but now there is a new company extracting profit, with overhead and all the related costs.

Is that a better world for the average person? I don't know. I just don't think the answer is as simple as saying DoorDash obviously makes it worse.