Makes no sense whatsoever. Clubs will be selling players for far less than their worth to other clubs just to be below the salary cap. And after losing those players, their revenue will fall again due to fewer jersey sales and they'll sell more players the next season. Not going to work.
And what if a team only uses 60% of their cap one season? Does the extra 40% carry onto the next? I don't think UEFA/FIFA wants to keep all those numbers around for every club there is.