Its really no worse than it was with keys. The flipper zero only works on very cheap, corner cutting simple systems. A lot of cars (and all cars should) use non-repeating codes so a simple interception is useless. That doesn't make them invincible of course.

Those cars would, back in the day, use simple corner cutting keys to be secured. There were quite a few cars back in the day that would have only a very small number of keys meaning there was a mon-trivial chance of you running into a car that you could open that wasn't your own. There are countless stories of people accidentally unlocking and getting into cars that are not there's.

Here's a concrete example, there are only about 5000 different keys for some brands of Toyota. A car thief could get 10keys and try 10cars a day (and remember this would take a minute or 2 and not really look suspicious) and successfully steal a car every 2 months or so. A dongle pretty decisively kills this avenue of attack. But like all things shitty engineering opens up new attacks, although on the whole it's a lot harder to steal a car today than before dongles.