The biggest threats to melee cav are: (in descending order)
Horse Archers, pikes, polearm users, archers(+throwers+xbowers), 2handers.
HA's and pikers are classes that the melee cav cant win over at all in a 1vs1.
Biggest threats to Horse archers:
Other horse archers, archers. (where archers are normally at a disadvantage, but they can still take down HA's)
As you can see, melee cavalry has a lot of "natural predators", though I still think throwing should get a little buff by reducing the penalty powerthrow makes on wpf. However, HA's have no real counters, except other HA's, which would then put them at an equal footing. (but getting countered by itself isnt really a counter)
So the title of this topic should be changed to: Horse archers have no natural predators