It sounds like it would work -- there'd be plenty of time when only EV drivers would be allowed to drive (since the quota isn't met at that moment). This would create a lot of incentive for people to get EVs, so that they could drive for Uber whenever they wanted, rather than only occasionally.
These conditions would need to be imposed on taxis as well, for maximal effect. Grandfather in existing taxis, and then past a certain date, new taxis must be EVs.
You're asking low income workers to suddenly front the bill for an entirely new vehicle. This isn't going to happen. What will happen is the same thing that did to people with the affordable care act in the united states: it becomes a regressive tax they pay because the alternative is simply too expensive. Better to pay the tax penalty than an unaffordable insurance requirement per month.
Most rideshare vehicles are financed, not bought outright in cash. If the increase in income from driving an EV exceeds the additional monthly cost of an EV then it's a no-brainer, especially once you factor in cheaper fuel costs.
And your take on the ACA is quite off; it's given healthcare to many more people than previously had.
These conditions would need to be imposed on taxis as well, for maximal effect. Grandfather in existing taxis, and then past a certain date, new taxis must be EVs.