I don't know quite what you mean about the AVR32 being a "poor step child" - it's an independent modern 32-bit processor architecture with all the advantages and disadvantages that brings.
But MIPS has been around for decades - it's older than ARM, and certainly much more established than the modern popular ARM cores. MIPS also have more experience at the higher end - they had 64-bit cores long before the amd64 architecture (or even Intel's disastrous IA-64), and SMP is standard stuff for MIPS. But they haven't had so much at the low end until recently - the microMIPS extensions are quite new. And they have always targeted a few big customers rather than being used by the "little people".
MIPS is a core that will never go away - it is far too established for that. People who want a CPU core for their chips that will be used for the next 20 years pick either MIPS or PPC cores.
I have no idea how popular the PIC32 is. Microchip's handling of "their" PIC32 compiler puts me off it.