I don't understand why a player can't just retire now that the back diving aspect has been removed. You could even put in some clause that in order to retire they need to ensure that they aren't paid what was left on the contract. So if someone got a huge signing bonus they would need to payoff whatever the difference between salary and cap hit was.
Marleau didn't add on a year to lower the cap hit, it's just weird that players aren't allowed to retire without crippling their team. This doesn't apply to those back diving contracts, really only to just any contracts signed since the new CBA.
*edit I don't think Marleau would have retired if this was the case, it's more of a comment on the current rules.