nobody has a problem with Castle age Poles. They are about as strong as, say, Slavs in Castle age. Trading cost-efficiently vs Camels is generally false because as per OP, you will mass some Knights in early Castle age paying the full Gold price, and even later on when you get the discount, you still need to recover the cost of the tech itself. So no, they don’t trade efficiently vs Camels. The fact that they “shred” other cavalry is also dubious, not sure what you mean here because Poles cavalier is inferior to a generic FU Cavalier, and in Imp, generally the game is a lot about holding & capturing positions, yes you might trade cost-efficiently (assuming no archers) but if you lose a 70 vs 70 fight, your reinforces run in 1 by 1 and you must surrender a position (e.g. Castle), so cost-efficiency here matters little.
many games go to Imp, in fact in high elo the majority of games last until at least early Imp. The thread is also about Imperial Poles gameplay, so I thought it was obvious that we assume somehow both sides made it through Castle age (e.g. 1 player failed the Castle age push, or the map is Arena, etc.)
there is no “infantry civ” per se, all the civs described as “infantry civs” really are hybrids which use 2-3 non-infantry units to complement infantry play. E.g. Celts → Siege, Viking → Crossbow, Japanese → Crossbow, Malay → Crossbow, infantry alone in this game sucks.