Do you expect that the old microcontrollers will continue forever (say the next 5 years) because they are so cheap. Or do you think that the new 32-bit microcontroller will become the de-facto standard?
The "old" 8-bit AVR microcontrollers will certainly still be in use in five years time. It's not so much about new technology as it is about fitness for purpose. There are still people designing and building things with PICs where the expense and complexity of even an AVR is not needed.
The future of the Arduino Due is less certain. It's pretty obvious that there are (and will be) an increasing number of designs using 32-bit ARM cores, but the further along this line you go, the less like hardware and the more like software everything becomes. If you are comfortable with writing code for an ARM CPU there are a lot of options, many of them cheaper than a Due. Raspberry Pi has a huge following; I happen to like the Freescale "freedom" boards a lot, too, but there are hundreds to choose from.