Embed IDML data into INDD files for permanent single-file offline backwards compatibility
Just embed IDML data into every INDD, at least optionally. A separate IDML file requires having foreseen the need for backwards compatibility, making sure it is up to date each time the INDD is changed, or resorting to server-side conversion, all of which are a nuisance and unnecessary.
An IDML’s file size is usually anemic when compared to its INDD counterpart, so that is a non-issue.
Even Photoshop asks you whether you want PSD files to be backwards compatible.