Yeah but we could always trade him in a couple seasons if the team hasn't progressed enough. The fact that Mantha is on board for 3 years pretty much tells me that he's not entirely committed to winning with the Wings. If this is the case, I would honestly consider moving him right now. If I'm Yzerman, and I get the feeling that he wants to sign for 3 years and then hit the UFA market, I would seriously consider moving him now while he has a ton of value. If Yzerman let it be known that Mantha is available, I can only imagine there would be a massive bidding war for him.
I would hate to sign him for 3 years, only to see him walk. I know there would always be the option of trading him at the deadline, but I don't think we would get a ton of value if we're trading him as a rental without an extension in place.