You might have people that want to drive taxis but they would still have to get used to the streets, how the app works etc. etc. which can significantly degrade service quality.
It isn't like other jobs that have resumes and (possibly) long interview processes.
From what I can find we are talking a few days without talking to anyone and you are driving. Throw in Uber and Lyft doing an advertising campaign with incentives to start driving, I don't see any reason they could not have a potential large amount of drivers fairly quickly.
Maybe it won't be at ideal hours, maybe it will still be hit or miss, but there are a lot of drivers out there. Just due to the very nature of this being gig work.
All they really need to just ignore the union's demands is to be able to sign up enough drivers to out last the members not making money. Getting used to driving the streets and everything is up to the drivers, not uber or lyft. I am just reluctant to think it will actually work and the drivers won't cave. Trying to pass laws would be a more concrete fix.
That puts the barrier to entry on the same level as grocery store workers. Granted, those too can successfully unionize; I agree that such strikes are only toothless when unemployment is high.