Contracts must expire
There was a client I worked for a few years ago who recently contacted me about doing work on their site, and I'm not able to complete work in a timely manner as they would expect. Over the years, things change - of course they change. It's impossible to foresee the change that will occur in the next year, but it will happen.
From change in my life, I've realized how important it is to have contracts expire, and to re-evaluate things. Almost everything in life needs re-evaluated as things change. Some things are daily, weekly, monthly, yearly. External factors in life do change, and you need to re-evaluate things.
When contracts are written up, they have to expire. Plan for the contract to be revisited in a few months, or a year - at an appropriate time. Things will change, and there's a good chance it will work out better for you when you re-evaluate old decisions.
