But... if the lords are free, you have more threatened battles. And we take all their fiefs too. I don't get your point.
There is a minimum amount of time a prisoner stays put, which is unarguably far too low, which makes the prisoner strategy the only (?) viable CONQUEST strategy. But this isn't a game of CONQUEST, it is a game of individual combat and individual achievement. Any amount of Conquest you get done is ephemeral, and that is *by design*.
I happen to agree with the design: crappy little kingdoms knocking the piss out of each other and then waiting in the shadows to make a comeback, for hundreds of years? That is realistic.
World domination is NOT realistic. But the game lets you do it anyway with a specific set of traits.
You'll have more battles, but they won't particularly be more threatening. Their best tend to get thrown at you first, then they'll have to pull some from their garrisons and start recruiting from notables, which should give decent troops as long as the notables haven't been drained of higher tier troops yet, then back into the
meatgrinder player's loving arms. Then it's downhill from there for troop quality until they build up their own parties more and the notables get a long enough break for their troops to level up.
I'm not really sure how you've come to the conclusion that conquest isn't something that TW wants the player to be able to do. The game's description mentions it, there are achievements around not only that but world domination, and it's a prequel to a game where conquering was supported and players were expected to do.
The game has mechanics that support it and put limits on it, like needing to make sure your holdings are stable unless you want a rebellion to wrest them away from you. Though I figure most players would just influence drain so that they could put +Loyalty policies up. I doubt that the devs put so much work into sieges for them to be a trap option that ultimately gets the player nothing. At least for towns, castles are meh. The work done to curtain snowballing wasn't so that no one could do anything ever, but that a non-speedrunning player could have more time before factions get too strong or weak. Though the general trend of clans toward Tier 6 means they'll get stronger anyway.
When I was talking about prisoner strategies, I meant that they seem to be the only viable way of getting positive tribute (maybe raiding too? Though that would come with its own costs), but that's not the same as conquering territory. For that it's just spilling a bunch of blood while avoiding getting too bled out to siege.
But yeah, you need over 40 before it even matters at all because they all have extra members to send out in replacement parties, so until you start cutting into those their amount of parties won't even change in a useful way. Even then they will pile on mercs and you have to catch them too. Of course you don't have to, I don't even have keen sight until I'm 1/2 done with the map these days, you can just doze them anyways and be annoyed with the conga of armies.