If Sano has the glove to stick at 3B, then I think you go year to year with Plouffe and plan on trading him in the next 1-2 years.
If Sano doesn't have a 3B glove, then Plouffe is miles away the best 3B in the system, right?
Someone mentioned Brandon Phillips (might have been the Dozier thread) as an example of a premature extension for a player who is now in decline. Has anyone looked at the Reds system? I count one middle infielder among the top 10 prospects - some guy named Alex Blandino with a 2017 ETA. Correct me if I'm wrong but nobody is challenging Phillips for the starting 2B job this year. Further, if the Reds hadn't locked him up when they did, they'd be paying more AAV to keep him on for longer years, or else gone shopping in FA and paid the market rate for someone else in decline or fast approaching it.
On the other hand, maybe a team like the A's were able to part with Donaldson because 5/5 top prospects are infielders, including two 3B's and a SS. Obviously they got Lawrie in return too so they are 4 strong in that dept. But, after Plouffe, the Twins really only have Sano.
Point is, if the Twins determine that Sano doesn't have the glove for 3B then they are stuck with Plouffe. Certainly for 2015-2016 and easily could be longer. Might as well lock him up while they can still pay a discounted rate.
edit: assuming of course TR continues to rely on the draft and amateur FA for acquiring 3B talent, and not trade.