I took a detailed look at workflow a while back, concentrating on the choice of file formats for image capture, processing, end use and storage.
At the time I was gravitating towards use of RAW plus XMP (sidecar) files for permanent storage, but I have since changed my mind. I now convert all my RAW files to DNG when I import them into Adobe Lightroom and regard DNG as the best long term format.
Having all your processing changes recorded in a separate file is messy and there is the risk of the RAW and XMP getting separated if files ever have to be moved around. Better combine them within a single DNG file. In effect, the DNG format brings the RAW data and the processing information in the XMP together under the safe umbrella of a single file.
And what’s the point of holding onto the original RAW file? The obsolescence risk must be higher than with a DNG. The format is becoming too widely used now to imagine that it would become endangered, even if Adobe ceased to trade.
Most importantly, the dependence on Adobe software to open and manipulate DNGs is reducing. If all else fails, the open source GIMP application can be used as a DNG-compatible Photoshop replacement. The fact that GIMP is open source should make it “immortal” or at least much less likely to die. If the vendor of a proprietary program goes under, the closed source code it owns would very likely disappear with it.
DNG is bigger than Adobe now.