Exactly! I've been mildly involved as my families archiver for years. PDF/A is the standard that the should be used for archiving docs. In fact it's what the library of congress recommends themselves for long term storage of docs. But as you said I can't think of a single app that does strict pdf/a support. I may have come across one it seems but can't remember what it was. I store everything in pdf and images as tiff.

The LOC has good guidelines for archiving all sorts of formats.