Reason 3: Establish an actual rebuild in Chicago
As mentioned previously, it is unlikely that Patrick Kane will win another Stanley Cup in Chicago. Sending Kane to a contending team would be the first big move demonstrating the Blackhawks are truly starting from scratch to build a serious contender. The mini rebuilds Stan Bowman constructed were not true rebuilds, and that is being proven by the product on the ice today.
Getting rid of all veterans with serious value and building through the draft is, in my opinion, the way to properly rebuild. Keeping Kane would in a way cause the Hawks to keep playing mediocre hockey for the next 2 seasons since there is no real cap space to get other needs taken care of. That also brings up a tough situation with a potential Patrick Kane extension.
As long as the Blackhawks are paying Patrick Kane and Jonathan Toews, they will not have the room to properly rebuild. Starting from scratch is a challenging and time consuming process, however, building the proper foundation is the only way a rebuild works. Trading Patrick Kane allows the Blackhawks to finally knock down the old broken down house they built for their dynasty, and start fresh with a new, strong house made for the future.