Actually, the chipKIT Uno32 and Max32 boards based on the PIC32 that are Ardunio compatible seem more compatible to Arduino than DUE. Most pins on the chipKIT boards have 5V tolerant inputs where the DUE does not. This means that most Arduino shield will work with chipKIT but not DUE.
Well, not quite so fast. The 5V tolerant go some way to ensuring shield compatibility, but there is still the issue of current limitations on gpio pins. Can the Uno32 source/sink 40mA on a gpio? I haven't looked at a data sheet, but I'd be surprised if this was the case.
But agreed, the 5V tolerant pins addresses one important compatibility issue. So certainly more Arduino shields could be expected to work with chipKIT compared to DUE, but "most" is not a given.
And whether you are porting your 8bit/5V project to Due or something like an Uno32, you will still need to tweak the code in any case.
I think we will see that the Due will be much more suited to new projects, rather than providing an upgrade path for existing 8bit/5V Arduino projects.