Transition to what, exactly? Pixie dust? Unicorn farts? There's nothing to transition to.
Elon Musk used an image of a farting unicorn to promote Tesla.
Not to mention that they're often burning stuff behind the curtain to power the non-burning stuff.
As long as it's out of sight it's okay.
It's still an improvement if the large place that burns stuff extracts more energy than tons of small places burning stuff. Not to mention, it's easier to then replace the few large places that burn stuff with something cleaner than having to replace every single car on the road again.
There are lot of parts in the network. No one claimed they would all make a step transition overnight. Part of that is to replace parts of the network which continue to work, while being capable moving the other parts forward.
From my experience, any car at all or even a bicycle could replace the majority of trips I see these trucks doing.
It's probably ok if the burning is happening off-planet, like on the sun.