Large space taken up by system files

My 2015 iPhone 6+ has almost 8GB out of 16GB taken up by System files—on my husbands 6+ iPhone it is 11.5GB out of 16 (the “grey matter “ to the right of the colored data in the iPhone storage bar). I am told by Apple support that these are mainly system files from older software that cannot be gotten rid of. Does anybody have suggestions on how to re-gain the storage capacity now blocked by the junk, please? Tx!