It is technically incorrect to make the blanket statement that a base coat/ clear coat system is not as durable as a single stage system.
There is more to applying a base coat/clear coat system, so there are more possibilities for incorrect application, and errors.
Having painted professionally for over 35 years, I have been through multiple, endless training, and warranty certification classes. And, I have spent time working directly with the chemists who develop automotive finishes, testing new products. So, I have a good understanding of what is involved.
There are lots of painters out there using base/clear systems, who either don't know the proper way to apply them, don't understand the chemistry behind why it's necessary to apply them in a particular fashion, or are not interested in doing things that way.
The nonprofessional, and untrained professionals, are almost always using lower end paint systems, which contributes to their issues.
Yes, there are also lots of vehicles which manufacturers, and their paint suppliers, who have messed up, trying to find the cheapest, lowest VOC way to get products out the door.
However, if applied properly, a base coat/clear coat system is just as durable as a single stage system. That's all vehicle manufacturers use now. And, durability is as high as it has ever been.
As further proof of that, manufacturers of high end jets are all using air dry, base/clear systems now. And, that's as difficult an environment as you can put paint in.