As our legal framework exists, no one can include the dissolution of a marriage in a contract. That said, everyone can, and these couples have, create your own private contract that includes what you are agreeing to do/not do. It’s not a renewal of vows — they agreeing to commit to new vows they’ve created that speak to where they are now and where they want to be for the next five years. They are not getting legally divorced (or for that matter legally remarried), but they are defining the goals of their marriage for the next few years and holding themselves and each other accountable for what they agree to. If they decide they no longer want “in,” then, yes, they will need to legally divorce. But because they know the whats and whys of their split, and have already agreed to the financial realities of splitting, it will be a lot less traumatic — financially for sure and maybe, hopefully, emotionally although that’s an unknown.
All that said, to create time-limited renewable marital contracts that are accepted within our legal system is the goal of my co-author and me — these couples are making it work for them within the system as it exists.
Hope that answers your questions.