As far as I can tell, workers are not being paid for just being online. They get paid a prorated amount of this "hourly" wage during an active order.
So the scheme above doesn't work too well for those folks but the economics for door dash remain the same. It also kind of implies you can create surge pricing for workers by having an overabundance of supply, which is a nightmare scenario for them.
The problem is that you cannot easily define people on call. If $33/hr is the average comp for trip time when just one person is working at $18 overall, then they're saying an average worker works 55% of the time. If you add another person who just idles on call for the whole hour, you must now pay the one worker guy $66/hr for his working time.
The game theory here is not going to work.
simple checks like "did this person do >X number of deliveries in Y amount of time?" should let the company know who to "send home" the exact same as food service and retail currently works. the companies are not forced to allow anyone onto the app, just as businesses are not forced to allow any employee who shows up unscheduled to sit around and collect an hourly wage.