I don't think he'll ever break 20 homers again. 28 was just an amazing year, and unfortunately for the Twins it was his contract year. I hate how people blame Mauer for the contract. If your boss came in and offered to triple your salary, you'd sign, no questions asked. You wouldn't tell your boss, "no thanks, I don't think i'm worth that much, i'll sign for less so i don't hurt the company." Just because Mauer is now making a ton of money doesn't mean he is going to be as good or better than he was when he signed. It sucks for the Twins, yes, but if anything you should blame management for offering the contract in the first place.
But, on the flip side, the Twins have been competitive and better team overall with a far less payroll than they have now, so even if Mauer is taking up too large of a % of the team's payroll, it's no excuse for the rest of the crappy team given that they've raised the payroll considerably over the last few years.