I agree with everything you said.
But single stage can be repaired just like base coat, you just have to wait
a little longer, 30 to 60 min usually.
It's a lot faster drying than enamel, don't compare to that,
they may both be single stage paints but they are totally different.
Urethane sets up hard as a rock, very quickly.
I paint in a garage and dirt is always a problem,
I can't use a paint I can't repair.
I would never argue that base coat is not easier, for sure it is,
and all the collision repairs I do I use a name brand base coat, Chromabase.
I would never use urethane for a repair.
And for any newbies I would recommend base because of it's easyness,
and being more foolproof.
That's not the topic here!
He's asking if he can clear coat urethane.
So that's what I'm answering.