A little late but I'll just mention that the "Decay" as you all have phrased it effects all attacks. It decreases both the knock-back and damage given. In Melee this was known as Stale Moves and only effected damage.
With Pit's F-Smash however it's not that big of a deal. It always does great damage and is fast enough to use in combos. Spamming it won't hurt knock-back too much and the damage it deals throughout the match will easily make up for the little knock-back it does lose.










