Now that he has signed the extension, Mauer is much more tradeable, not less. Now a team wishing to acquire him doesn't have to negotiate an extension. The work has already been done.
There is no way he's getting traded. Dude is beloved here. He would have to completely fall apart for the Twins to think about moving him, and if that were the case, no team would would want him at those kind of dollars.
I don't think people outside this area have any idea what he means to this area. I can't think of any player that is more beloved by his local fans that Mauer is. The love for the guy verges on creepy. If he left, attendance would plummet, even if the team was still good. He brings in so many fair-weather fans that otherwise would have nothing to do with baseball.
This would have been true whether he signed an extension or not.
Now his value is fixed. He reportedly has a no trade clause, but I have no doubt he would approve a trade to the Red Sox or Yankees for a chance at a ring.
Wll MN ever trade him? Who knows. It depends on what is being offered. If the Sox offered Lester, VMart and Papelbon (just as an example) the Twins would have to consider it.
The Twins' pitching is not championship caliber, especially with the loss of Nathan. If they traded Mauer and the result was a championship team, the fans would come.