Note that the universal wallet format does not need to equal the internal wallet format used by the software. Wallets can store their internal database in any way that is efficient (and secure) for them as long as there is a way to import/export the universal format.
Sure, that's why I called it an "export" format in my post.
Bitcoin Core supports the `dumpwallet` and `importwallet` RPC commands for going from/to an interchange wallet format. This is basically just a text file with a list of private keys with metadata.
Not that useful until there's no common "Import Bitcoin Core wallet" feature in Electrum, Armory, Android Wallet etc. Blame me if the feature exists and I couldn't notice.
BTW: in general, unless you have a very good reason to do otherwise, I'd recommend to generate new keys and transfer your coins when you switch to a new wallet. This is safer and better supported.
I know what you mean and I agree on safety, but the need for universal formats certainly stands in backup scenarios where relying on an individual developer choice to understand my own data is highly undesirable. Many formats are badly or not documented and I'd rather refer to an accepted document to parse a wallet file than asking the software author or a forum.
Think of a "philosophical" problem: a wallet should mean something per se despite the existence of a software that uses it.