Players age; they're not as quick as they used to be (or in some cases they're even slower than they were); it's a natural progression that every side goes through.
Transition is never easy unless you throw obscene amounts of cash at it, so we've always got to have one eye on the future. Over-reliance on players who aren't getting any younger is only going to store up a lot of trouble.
Yes, those players won the Cup, but as a side we've been deteriorating since then simply because of this over-reliance on the Cup winners who aren't getting any younger and who are clearly visibly slower this season. We need to move on, and by playing the younger players regularly we will do that. Stay with the same old same old and we will stagnate before being relegated. Onwards and upwards.