Seeing all this makes me hope for a mod that makes troops much harder to level up and pay. Its clearly not what TW is going for, and I don't imagine all too many players would like it either.
Sorry for mentioning Warband again, but it has quite a few mods like this.
In practice, slower leveling _in itself_ doesn't work. Players will soon realize what units you need, to win battles with the minimum possible casualities. In every mod, this means top tier units, because they are the ones who have the armor and the DPS.
When units are harder to level up, but the optimal strategy is still to get as much top tier as possible, it means only more grinding. More fights with looters and bandits before the real fight.
If you have Warband, fire up Brytenwalda or Viking Conquest to see the effects of slow leveling. (spoiler: not fun at all)
But dammit, I want there to be a reason for lower tier troops to exist.
Now this is a very good point!
I don't know many Warband mods which make lower tiers useful, but there are a few:
- The Last Days: Orc infantry is the "zerg" of the mod. They are quite bad in every way, but they are very cheap, don't require food (you can feed them prisoners), and you take three orcs per one army size. (= with an army size of 100, you can have 100 humans or 300 orcs)
- 1257AD: recuitment is not by person (eg. 1-10 recruit) but by lance (eg. 1 knight, 1 squire, 5 crossbowman and 13 footman at once). Knights _are_ much better than the rest. However, you cannot get a knight alone, he always comes with his supporting footman and crossbows. Since knights are much more protected than the rest (=the footman die more often), there is a good chance that during a long game the percentage of knights in your army will be higher. Maybe 1 in 5, instad of 1 in 20 you start with. But it is impossible to get 100% knights because of the recruitment method.
- No cross-class upgrade mods (most multiplayer mods with single player campaigns): multiple troop trees per faction, and there is no mixing between them. So, there is no "greek recruit", but greek skirmisher recuit, greek hoplite recruit an greek cavalry recruit. You have a random chance to get them in a village. Random chance, but it is a guided chance (romans have a higher chance to get infantry than cavalry or skirmishers)
It is a good question why would romans use auxilia spearman? They get the nice legions, heavy infantry in shining armor. The first time you are charged by gaul cavalry, you will realize that spearman which cannot advance past tier 2 is still better, than trying to stop a charge with tier 4 legions, who don't have spears at all.
When all unit tiers have some kind of use in the battlefield, which is not replicated by the top tier unit, there is an incentive to get them.
- With fire and sword. The speciality of this game is when you get a level 1 soldier (from castles or mercenary camps), it is a drilled soldier, and gets the job done just fine. The higher tiers have bit better skills, have a chance (but not guarantee) for better equipment, and of course have higher wages, but there is usually no rush to upgrade, as the bottom of the barrel tier 1 units are already pro soldiers.
It is not necessary for low tier units to be bad.
- Native Warband: actually there is a case where I don't upgrade units. The thing is that vaegir infantry drops the shield in higher tiers. I mostly recruit vaegirs to get marksman, don't really care about their infantry. And actually lower tier infantry, which comes equipped with a shield protect the archers better than vaegir guards. The other case in native warband is the khergit khanate, where there is not much difference between horsemen and horse archers, so I don't prioritize upgrading them.
The role of different tiers might change, so it makes sense not to level them up.