Something that has always bugged about MTW is units becoming obsolete & no longer buildable.

Historically certain styles of warfare and the troops thus required die out due to factors such as economics, battlefield effectiveness - whether perceived or real - military fashion & demographic changes.

Would the Saxon/Scandinavian style of axe-wielding heavy infantry, the huscarle, have died out if William had lost at Hastings. If the Normans hadn't had the confidence and 'proof' of the apparent superiority of their style of heavy horse over the huscarle from that battle, would the eventual rise of the Western knight with couched lance - something that owes a lot to the Normans - have occured or been as widespread? Further down that line of thought, would all the countertroops, such as the pike, have then been developed/re-developed?

By the nature of the game, history in MTW does not follow the same pattern as our real history, so why would your military stop arming themselves and fighting in the style of huscarles, for example, if you win battle after battle with those troops? The gradual technology changes that improved the armour & weapons of the knight would in this alternate game history have been instead applied to the huscarle, perhaps eventually leading to them being plate-armored - in effect the chivalric/gothic foot knight, but via a different route.

With this thinking, I prefer to mod the game so that for campaigns that begin in Early no troop type ever becomes obsolete, leaving it to the whims of the AI to determine what gets built by the other factions. With High and Late starting campaigns, I consider the history to be fixed and troop types that are already obsolete stay obsolete though. I haven't yet gone as far as creating new units that are perhaps logical developments of otherwise abandoned troop types, the plate-armored huscarle being an obvious one from my above example, but it would make an interesting alternate-history mod. Maybe one day, but with RTW on the way, MTW itself will soon be obsolete.