Man y'all need to stop rewriting history.
That is intentional. To mitigate the consequences of kingdoms being defeated and conquered.
This change was first made to let player kingdoms actually recruit troops. When this was still a limitation you would start a kingdom and only be able to recruit from your few towns which completely ****ed you over unless you created a kingdom with a massive amount of land. It legit would ruin a playthrough because you also couldn't abdicate your kingdom like you can now and you'd be stuck with like 3 parties trying to recruit from 3 places while fending off 1000 men armies.
While I can see some validity to that mechanic, imho there should be some kind of penalty to recruiting outside of your faction especially when done to factions your at war with. Maybe excessive recruitment of enemy villages reduces morale greatly or tanks army cohesion. But to freely allow rival factions to recruit without some kind of deterrent is a bad game play mechanic. This is part of the reason why factions bounce back so quickly, when a faction starts losing ground they need to sue for peace and try to turtle up until they can recover instead they just keep recruiting anyone and everyone to throw into the grind mill. It's so shallow.
AI can't recruit from enemy villages, i think it was a bug in the past but has been fixed (same as them being able to recruit from raided villages, also fixed).
If you actually look at AI parties you'll see they are like 95%-99% homogenous (just go look at my post about noble troops) until a kingdom expands it territories and lords start to own land that isnt their same culture. I really dont think this is an issue like you are claiming. I'm also almost positive that most foreign troops come from recruiting prisoners to the party.
Taleworlds often looks for the easiest fixes to complex problems. They could've gone with alliances for example to fix snowballing but they opted for something simpler. They make a simple change and call it a day without thinking about any ramifications and then you get what we have now where battles don't matter, where troops are just a limitless pool like blades of grass on a prairie or where fortifications like castles have zero value to anyone because there's nothing inside or out of them except a place to occasionally do battle or to store some extra gear.
My going theory on why people think there is a limitless pool of enemy troops is that player involved kingdoms dont seem to really raid villages and raiding villages is really the only way to stop an enemies force from easily rejuvenating. Can't prove this one yet for when a player is just a vassal in an AI kingdom, but from my experience in a player owned kingdom, the AI vassals are to busy joining armies to actually raid. But the over creation of armies is tied to having high influence which can be tied to using to many influence policies. AI kingdoms dont seem to have this issue and will raid villages like normal.
Also maybe people don't realize that a faction is likely to have 3 to 4 standing armies?
They've simplified the world mechanics to idiotic level in the name of "yeah, but Snowballing...". Its their job to figure out new innovative and robust systems to counter the dreaded "snowballing". That Snowballing thread annoyed the everLivin Jesus outta me because it was all quick fix type stuff like this -they need a fine surgeon's scalpel not an idiots sledgehammer.
KEEP MY THREADS NAME OUT YO MOUTH
That thread was the only reason we got any solution at all. If you read through it you'll see we all suggested the innovative and robust systems like below (even MostBlunted before he was reformed) and our man agreed, took it to the table but it got rejected by the one who has lost his way.
Yes, actual diplomacy. Treaties, alliances, non-aggression pacts, faction relations etc. would have been a far better way to stop repetitive 'snowballing'. I still don't understand why they decided to have zero diplomacy in the game. I'm guessing it's something to do with consoles.
We all need to blame the real problem (like my signature).
not that it will do anything