Witten undoubtably is a a good role model for younger players. I disagree with your assessment of his current ability to block, but obviously that is a matter of opinion. You don't pay a player to be a good role model. That really should come along with the player as a package deal. If you are wanting to pay him to solely be a role model, then give him a coaching job, which doesn't count against the cap and pay him as much as you want to.
But as a player his cap number (and his salary) are overly inflated vs what he provides at this point in his career in terms of production. You really owe it to yourself to watch him block instead of relying on your memory of the "good ole days" when Witten was an effective, consistent blocker. Those days are long gone. His blocking effort really is actually pretty lackluster too, if you really want to get to call a spade a spade..