[michaelt]

Assume you work on Keynote, so you have access to all the code and documentation for the 09 format. On average computers are now faster and have more RAM. You don't need to write the format, only read it. Your users will tolerate 15 minute format conversion times and the loss some things like formatting and videos.

Is it really so difficult to just not break a feature that was present in the last version of the software?

[TillE]

Yeah, assuming your software is reasonably modular and you haven't completely rewritten everything from scratch in the new version, retaining support for old file formats shouldn't be a very big deal.

At worst (like in the case of iWork 13 which probably is completely new code), you get one programmer to spend a few days writing a converter which reuses most of the old code.

[XorNot]

More importantly, provided you don't do it too often, users will tolerate some loss of formarting fidelity from version to version.