I agree, but an an alternative to mbox/maildir format on your machine : I wrote https://github.com/karteum/gmvaultdb in order to have a single-file SQLite DB with all old email contents (all UTF-8 inside, simple/minimalistic schema), together with a small GUI to consult emails and do queries (full-text not fully implemented yet but this is the target) and all attachments extracted in a side folder so that I can directly browse with a file manager.