The "files" are really a folder/bundle with richtext and image files inside it. Why doesn't OP just extract the actual data from inside the bundle by right click > package contents
FFS, this is "HackerNews"... write a bash script to do it for you. Better yet, use some "super 1337 h4x0r google-fu" and search "convert keynote 08 to keynote 09" and you'll find a bash script to do it in 5 seconds[1].
[1]: https://www.google.com/search?q=convert+keynote+08+to+keynot...
I have written quite a few bash scripts, python scripts and long ago ruby and smalltalk scripts to do things for me, that bothered me. My point is, this is a software I am paying for (and I've already mentioned that 2008 should not be considered as old) and I would rather spend my time writing the "super 1337 h4x0r" bash and python scripts for something that I can't do otherwise.
Apple leads users to expect an easy experience, and not to need to worry about technical details. The cost of that strategy is that Apple should expect criticism when those expectations aren't met.
But this makes it even less clear why Apple can't support their own old formats. Because it's too tricky to be worth dedicate developer time?? Or because it's a deliberate policy? This is a fundamental requirement of the software.