some things work with small processors, some don't (neural nets, speech recognition by FFT cross correlation, full-HD multimedia,...)
Using libs on the PI like joan's pigpio, you may work already extremely quick on the Pi's proprietary GPIO pins, and using pthread by thread priorities you can come quite close to real-time behaviour - not to talk about real-time kernels.
OTOH, attaching additional hook-up AVR- and ARM-boards (e.g., Propeller-HAT or even additional MCUs integrated on the backpane as on the BBBlue or once planned on the TRE or even on the YUN) you can have the best of 2 worlds.
All depends on what you intend to do, no question.
For a hen flap a AVR will do, for a neural net a 64bit 1GHz 1GB quadcore might even bend your knees...
The IDEs and the API libs will make the difference in the end.
And not to forget: I am finally not known to be a friend of Linux or Python.
So why not have a bare-metal 64bit-super-quadcore powered by a Arduino IDE?
Or perhaps even driven by Windows 10?
But back to topic:
The Due is highly neglected currently IMO, but eventually: it's already available, and so it's actually not a long way off.