You really have no argument until you can tell me who you want as that top 6 center and how the Rangers would deal with the losses of the assets to get this unnamed center.
Trading Staal and then "finding a guy to play 2nd pairing LD" is not a suitable answer. Its glossing over what could become a major issue.
Who I want is not necessarily available. There have been some rumblings that the Wild are willing to part with one of their young players (namely either Coyle or Granlund) to get a more veteran defender back in return. From what I've heard, they're trying to pair that veteran defender with Dumba so it allows him to grow and allows the team to have a more solid defense than they went into the playoffs with this year.
So ideally, I'd try and swing a trade centered around Staal and Gralund. Obviously, we would be entitled to more, such as a 2nd round draft pick or a B level prospect, considering that Granlund has only played 60 somewhat games in the NHL and Staal is a much more proven and dominant commodity.
The one year rental thing is justified, but then again Minnesota is a team that has had excellent success attracting and retaining high-end players.
Ideally, I'm targeting something similar to what you were advocating for earlier in the year - young, cost controllable, talented players for guys that are getting older and unaffordable.
Staal creates a hole, sure. That hole could still be evident next year. He's going to want 5.5+, by all accounts. Is that a good allocation of funds in cap-run league? 5.5+ for a 2nd line player? 4.5+ for a 3rd line player in Brassard. No matter how dominant those players are, is that really proper value? And if the Rangers determine that its not good value for Staal, they can either lose him for nothing at the end of the season while he walks, or they can trade him at a highly diminished value because he'll change from an 82 game rental to a 15 game one.
Personally, i'd try and trade Staal for either Granlund or ROR, the latter being the more unappealing to me. Hell, I'd even be willing to trade Staal for someone like Mantha in an attempt to add size to the wing in place of Poo.
Maximize value. That hole might still be there in one year's time, even if we don't trade Staal.