There's a max of 15% per year. But, since the dues are almost entirely based upon the operating costs of the resort, you can probably expect a 3-4% annual increase over the life of the contract. That's just the cost of doing business.
The most damaging thing Disney could do would be to schedule additional maintenance in the waning years of the contract, but they already maintain a reserve fund for that purpose.
Anyway, annual increases on that order will probably see the dues reach the $17-18 per point range in another 50 years (for SSR). While that may sound a bit shocking, Disney has been increasing cash rates at its hotels 4-5% per year. A $300 room in 2005 will cost over $2000 per night in 2054 assuming a 4% annual rate increase.