Love the idea that people can project how much parking there will be in 75 years. It's possible there won't be cars at all in the areas parking meters currently make most of their money in Chicago.
Presumably any explicit policies along these lines would require the "true up" payments described in the article. If you mean a non-mandated move away from cars in that time, the only plausible scenario I'd imagine that happening is one where Chicago has much more serious problems than a bad contract.
It's possible, but as others are saying, there are guarantees in the contract. All things being equal, this makes it more likely that Chicago will do what it can to motivate people to park cars next to meters for 75 years to come.