This is a difficult one because Processing was originally designed for non-programmers who would have struggled with OO concepts and syntax. The pre-processor hides the fact that Java is an object orientated language and gave beginners a simple syntax to write their sketches.
I do wonder how many programming newbies would continue with Processing without the pre-processor simplifying the language syntax.
Most of Processing and a huge chunk of Java is not thread safe. This is not a problem for the majority of Processing sketchers and for experienced programmers Java provides mechanisms for safe threading.
My personal interest in this area is G4P where all the controls have their own internal graphics buffer using lazy updates to improve performance. I should point out that G4P operates entirely within the main event thread with the exception of multiple windows where each window is effectively another Processing sketch.
I am currently working on the next version of G4P and I was creating an example sketch in Processing to demonstrate a new control when I hit the problem.
To be honest, I had come across this problem before but had forgotten about it. C’est la vie 