I think cav itself isn't really OP (at least it has to be a bit OP as it is cav!), just the amount of cav is a problem.
I do think the game should be balanced so same amount of upkeep is balanced.
It is fine that people can empty their gold storage slowly by playing heavy cavalry with heavy armor and be stronger then people spending less.
But if you think about the difference between being on foot or on a Rouncey for example it is huge with the right weapon and skill.
What I am trying to say here is that imagine a graph for each 3 class. Where you would put upkeep cost on the X and K/D ratio on the Y.
The graphs wouldn't look anything alike.
Infantry graph would go fast up but once you reach about 10k it will go really slowly up and probably even down once you are forced to wear heavy armor to make that upkeep.
Archers graph would set the power pretty much all around the same amount of upkeep. Since they have no real options anymore what to pick. Play like this or don't play an archer.
Cavalry on the other hand would always go up with higher upkeep and the peak would be higher then the other two for sure. Which is fine if the game makes the neutral zone be balanced.
In short : The solution to balance is to make every classes peak at the 0-50000 gold upkeep zone to be the same. Everything higher then that can't be supported without giving something up (Loom points or negative on your vault). Best would obviously be to make it actually valuable for all the classes to be near the upkeep zone so we see fewer players get huge vaults.