Why would an implausible scenario need a plausible alternative?
Because the Warren Commission's "implausible scenario" is a lot less implausible than any of the anti-CIA / anti-FBI / anti-Military scenarios that you and the other tinfoil-hat JFKA conspiracy theorists have been able to come up with over the past sixty years, especially given the fact that most if not all of your conspiracy theories require an implausibly large number of bad guys involved in the planning, the "patsy-ing," the shooting, and the all-important cover up.
D'oh