Course he won't, Mclaren aren't even slightly interested in the constructors world championship and the simple fact that he can beat Button in every single race because he'll out qualify and out drive him in the race is neither here nor there.
They don't "favour" him, considering they've been letting Button use Hamilton's settings because Button apparently has no idea how to set up a car that doesn't have a huge range of exhaust, engine mapping, double diffusing cornering aids, I would say they've been spending more time helping Button match Hamilton than giving Hamilton loads of advantages.
Fact is he works for them, a new contract for next year is just that, he's still being paid by them and Hamilton is still making money for them, more for every point he gets, much more if they can get the constructors championship, more if Hamilton can sneak the title(though that is stupidly unlikely at this point). Mclaren will try to get him every point he's capable of.
That is of course assuming he goes, confirmation from more than the Sun is required. IF he does go its the right decision, Mclaren might go on and win the next 10 drivers titles, history and experience rather suggests that the team at Mclaren currently isn't a team that can win many titles. Merc aren't doing half bad considering they've had a significantly lower budget and team size for the past 3 years and its only just up to size. Surely Merc with 50% more staff and a higher budget will start to improve in the next season, maybe two and there is the simple other fact also... I think Hamilton/Alonso/Vettel in a Merc THIS season would have significantly more points than a Rosberg anyway. For one, it would mean one of those drivers not getting points in a top car, with a worse driver in one of those cars. If Alonso wasn't at Ferrari and another driver was matching Massa's performance, its likely a Merc with Alonso in would both have qualified higher and done much better in races.
In other words, Merc are probably faster than they appear currently as they don't have a genuinely top driver. I don't think Alonso/Hamilton could push for a title in the current Merc at all... but if the gap is closer than it appears currently, its also less of a gap for Merc to gain on the front guys than people think