I understand your complaint and your fear, but I don't see the point of worrying about it now. It seems like playing the fortuneteller game. How do you know when/if the contract is going to end up being bad? If you trade too early, and the return sucks, then what? How is that not also a fear?
Why even worry about something like this???
In general you will always appear to be overpaying for top players on a long term deal, especially early on, and at the end of the contract there is always risk which is why they WANT long term deals. If you don't pay that price you never retain or attract talent.
You have to hope they turn out to be a bargain later as the cap goes up and salaries continue to rise. You also have to hope you get plenty of value early so the last few years can be cost-averaged with the rest. You don't pay only for exactly what you need every single year like some kind of lunch menu.