If it keeps being so when K-mod is implemented a hard cap would be good. It will help to maintain a balance of power better. It lets smaller nation survive longer. From vassalage it becomes this snowball effect. Think about it, choke points would be better to defend. You would get more of a frontline.
In the last two (SVN) games i have seen nations with acces to horse only use mounted units, both light and heavy, exclusively for offense. While having only a couple melee and the usual city defenders. Two reasons speed and power. Thereby the light and heavy horse complement each other well, good against almost all units *if put together, but that is not the problem. The AI just chooses the strongest units.
Now with light infantry(gunpowder) i see them in combination with cuirrasiers (both 2 movement points). Since the defender is having an advantage over the agressor in home territory The AI opts for range. This has the effect of having limited amout of diverse units in AI stack.
I would advocate 2 base movement for all units with 3 for cavalry. That with a stacking limit that goes up with technology, with cities a bit of an exception. City defenders would then have the chance to sally out the castle and return as it is now only for cavalry (without roads). Hopefully this will make combat more dynamic.