Even multiple cores aren't the golden bullet some people seem to think they are. Multitasking, multiprocessing, multiple cores, multiple boards, whatever.
If they are on the same project they have to work together. There will be a shared resource of some sort or it isn't the same project. Typically multiprocessor systems share the same RAM and the same hard disk. So now you have to compete for that resource. You need critical sections, semaphores, locks, unlocks, etc.
This introduces complexity. It also introduces the possibility of race conditions, where A waits on B and B waits on A, and nothing happens.
The fact is that the Arduino can do quite complex stuff. For example, I've done VGA video output which requires precise timing. Others have done line-following robots, beer brewing machines, helicopters, etc.
I've done an Adventure game where the processor accepts input, and displays information on the screen.
If you think you have to replace your Arduino because you need to do something more complicated than blink an LED, you haven't understood how to make it work, that's all.
There are some limitations, there will be with anything. The basic speed of the processor means you won't be doing 3D graphics in a big hurry at high resolutions. But what do you expect for $5?