I don't think salt being made permanent makes much sense. If it were, the mines would barely ever reset, making it nearly impossible for anyone to start from scratch. Also it would be easier for players to just wait for the mines to level up for chances at rares without doing any of the 'work'.
It's not a super complicated system but it is a system. If anything, it would get replaced with a different system down the line instead of being made permanent. (100% speculation from someone who has not been paying attention)