That a later version of a software breaks compatibility with and older version is quite “standard” I’d say. What Apple could do, as a “bonus” or what ever, is having a “save as version X” and then just discard whatever features that are not avaliable in the previous version.
As someone stated though you CAN do this, sort of, with the XML-export (worked between 4.5 <-> 5.0 for me on a project perfectly).
No-one know if FCP6 is just around the corner