Cavalry is nerfed to be on a level playing field with other classes (imo it's worse off than infantry, but then, I play in a different arena to most c-rpg players), so it makes sense to nerf the upkeep to something comparable. The original idea of upkeep was to let people play in high tier gear only when wealthy from using low tier gear, but what we have now is two balance systems that seem somewhat indipendant of each other and a few classes are much worse off for it.
Never ever see plated archers these days, though they're often shown in medieval manuscripts and were a pretty cool part of the mod back when upkeep wasn't included in the balancing factors. Of course I hated them in person, but it's a shame to see them gone. They're gone because they are subject to upkeep and they're also subject to all of the things that kept them to a minimum before upkeep.
Cavalry is still viable, but it's reduced to killing peasants and the unattentive, which seems like a pretty pathetic niche for a class with unrivalled potential upkeep.