I actually brought this up in another topic. I believe the root cause is that Spearmen and Crossbowmen, as counter-units to Knights, cost too few resources for the same population. This prevents players from massing forces effectively, leaving them constantly whittled down by Knights.
I believe the fundamental solution is to introduce a somewhat expensive, highly effective, yet hard-countered generalist anti-cavalry unit with a lower cost-efficiency, like the standard Janissary.
You can check out the discussion over there for more details.