shrug  Not everyone that uses DS has DS3, either.  It may not be fair to the people who haven't or can't upgrade, but demanding backward compatibility forever is just not realistic.  There are more and more features in products where, if it does work in the current version, it should work in previous ones, but there's no guarantee.  File types and structures haven't changed, so that should have no bearing on compatibility.  The way P8 reads the files can't be that different from how P7 reads the files.  
It's about priorities, and not shoving so much work on their QA team that more mistakes get through than not.

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Knowledge is knowing a tomato is a fruit. Wisdom is not putting it into a fruit salad.