As far as I understand, converting input files to DNG is reasonable for Raw files only. Or is there any benefit if I convert jpgs to dng?
One hidden benefit of using it on a JPEG is that it converts it to TIFF which is, often, a lossless storage format.
HOWEVER, the TIFF standard supports compression: LZ; zip; and JPEG. LZ and zip are related and are lossless compression methods, whereas JPEG is designed to throw away pixel data to arrive at smaller sizes.
It is unclear to me whether DNG readers can be counted on to support LZ (Lempel Ziv compression). Old TIFF standards had it specified, but DNG appears to reference a newer "TIFF/EP" standard which specifies only "lossless JPEG" (whatever that means!) support as a required feature of a reader.
Presumably the use of compression is configurable.
For the uses that DNG is intended for, I would expect that bit-perfect is more important than storage space especially in light of the ever-dropping cost per gigabyte.
Note also that currently DNG is open source and royalty free and is "unencumbered" by prior patents in the development. This often encourages longer software support time and even 50 years out, the source code will be available for your grandchildren to compile should they want to see old photos of you from MyFace