I imagine it would cost a fortune to install enough readers to make them hard to avoid, and find it hard to believe that there are all that many trips that stay wholly within the zone.

Most people in the zone don't own private cars. I live on the LES (a peripheral part of the zone) where a lot of people DO own private cars, but they're almost exclusively used for trips out of the zone because within the zone there are a ton of ways to get around (public transit, bike/Citibike, taxis/FHVs) that, compared with needing to park a private car, are at least 2 out of 3 of cheaper, more convenient, and faster. I own a car and VERY rarely use it for trips wholly within the zone. The only person I know who regularly drove within the zone was a neighbor who, when his kids were little, would drive them to school in the East Village. But he wasn't driving in particularly congested areas and was only on the road for 10 minutes each morning.

Because parking is so expensive, there are also very few businesses that keep vehicles in the zone. More and more deliveries are done by bicycle or cargobike, and deliveries that require a truck tend to originate outside the zone. The biggest commercial vehicle fleets I can think of within the zone are UPS and FedEx, which both have giant garages on the west side, but a $20/truck/day toll isn't going to change their behavior (each already pays many millions a year in parking fines), and I'll bet a big portion of their vehicles leave the zone at some point during the course of a day to make deliveries uptown or in other boroughs and so will get hit with the toll anyway.