Just wondering,what would it take for light cavalry tactics,especially horse archery and Parthian shot,to be the primary military doctrine of most feudal European armies(especially the ones from Western Europe) instead of heavy shock cavalry tactics like couched lance charge?
The trend since late Antiquity had been that mounted armies in Europe were heading towards the effective use of heavy cavalry as a primary offensive force. The development of the Roman cataphract in both the east and the west was a testament to this period trend. Further-developed cavalry spathas became predecessors to later medieval one-handed cavalry swords, including the Frankish/Viking/Slavic/etc. cavalry arming swords of the 8th-11th century. Germanic cavalry that inspired Roman cataphracts evolved over the course of some three or four centuries into Frankish proto-knight cavalrymen. (And, ironically, the Frankish cavalry also borrowed some conventions back from the late Roman cataphract model.)
To reverse the steadily rising prominence of heavy cavalry in late-ancient and early medieval Europe would require some very good reasons, but it's hard to find plausible ones. Unless there is some major societal/ethnic/political upheaval throughout much of the continent, I can't see light cavalry gaining (or regaining) a primary role in mounted combat.
Also, watch this video if you're convinced that horse archers would pwn everyone. They wouldn't. Ancient Magyars learned it the hard way. The Mongols in the 13th century were only as successful as they were because they had extremely good organisation and discipline in their army and amassed a gigantic force by the time they reached Europe.