So if the term did apply then loop() would be a subroutine.
So when you use the return statement in loop() it returns to main(), does the stupid serialEventRun() thing, then starts the next for loop, which calls loop() again. That takes you back to the top of the loop function.
You exit the program. Since there is nowhere to exit to on your microcontroller everything just hangs after that. Give it a try.
Everything -- so nice! so beautiful!! so logical!!!
+!