The Supreme Court decision actually mentions that the government may "ask" car makers to just give them access to the car's own GPS device. Scary that the Supreme Court even suspects this will happen.

I don't get why the police didn't do this to begin with and then they could have simply blamed the auto manufacturer for not requesting a warrant.

I think all you need is a netwrok of surveillance cameras, which is already in place, to varying extents depending on where you live.

Then it's a software challenge to automatically read the licence plates in view and keep track of them in a database.

Or perhaps following the car with a drone.

You're over thinking it. Red light cameras can already read the license plate of every car that goes through an intersection. The roving fleet of meter maids in DC drive cars that read the plates of every car they pass.

Until we have drones smart enough to focus on a specific car (vs. a mostly identical model right next to it), the drones will still require human operators, which makes the process not far removed from an old-fashioned tail (similar to the mentioned beeper case).

Targets can be laser painted/marked by a drone or more usually a secondary source and followed by a drone on the specific signature. This is currently actively being done in several combat theatres.

It is highly likely to be already happening in the US, particularly along the borders and in federal investigations.