>What would it look like to have a thriving ecosystem of third-party clients for Google Docs style word processing, which can all interoperate with each other, even supporting realtime collaboration? Well, based on decades of history of other complex file formats such as pdf, zip, and MS Office formats of doc, docx, xls, xlsx, etc... it would be a buggy mess. It didn't matter whether the format was reverse-engineered…
As for the mbox problem: on a normal machine that must be shared between multiple tools (at the very least: biff,lmptpd, and the MUA) if thunderbird can't interoperate that's a serious bug.