For digital preservation, I find the following optimal:
- Shoot RAW + JPEG.
- Store both files.
RAW is as close as possible to what the sensor sees. Even PNG has some losses, including losses from denoising and demosaicing algorithms. RAW has none of those losses. For digital photography, anything including denoising and demosaicing is not lossless.
However, RAW requires a capable software to interpret the data. It is not an image, it is raw data from the sensor. It becomes an image only after interpretation.
Software changes over time. Thus, the interpretation of RAW changes. It will probably improve over time, but some changes may actually end up being negative for some rare images. Also, can you after 30 years find a software for the operating systems used then for a 30-year old camera? Perhaps, perhaps not.
By storing JPEG too, you are ensuring you at least have something to work with if the RAW image requires too much effort to convert to a lossless format using the software of the future.
The JPEG takes minimal additional space compared to RAW.
As already explained, there is absolutely no benefit to convert JPEG to PNG as the losses are already there. JPEG cannot be converted to RAW in practice.