TOO STEEP PATH is the result of constraining waypoints altitude wise followed by a speed reduction/restriction, and the FMS has computed that is cannot dissipate the Energy to loose Speed and Altitude (unless you help him with a liitle bit of speed brakes)
Yes, I understand the above however I disagree with the FMS because we actually can loose both speed and altitude without any need for speedbrake (we can do what the FMS says we can't do). We get these messages all the time and disregard them as we know what the aircraft can actually achieve (without any speed brake either) compared to what it computes what it can / can't achieve !!
For example, the profile below (descent gates) are easily achievable (open descent) however the FMS will always say "TOO STEEP PATH".
250 knots, 8000' 30 track miles
250 knots, 5000', 20 track miles
green dot, 3000' 10 track miles
Will modification of the idle factor resolve this problem ?