What is Fedora?

You are right that handling the ABI aspects would make this a major and long term project, involving a lot of upstream work. Albeit I think a worthwhile project. The autopackage people did a lot of good research into this; just a shame about their package format...

There are other aspects too - some people might have objected to an automatic upgrade e.g. from OpenOffice 2 to OpenOffice 3 when version 3 became stable, so that (the extra work of) maintaining both in parallel would make sense. This would probably be more relevant to distributions like RedHat than to Fedora though.