There is no logic in the ability to buy a pikeman 1929. One way to change this is to let LK have its own upgrade path so when you hit riflling you instead buy another type of mercenary (riflemen replacement) and of course you could upgrade your LK to this. And then also a GW infantry replacement and so on. So if you have taken this SP you will always during the game have a mercenary available of the latest type that is known to you. Although I do think they should obsolete them when you hit the Industrial Era or something or even with the Lancer kicks in.