In this game, unit get obsolete when upgrade unit is available, regardless if you can or not produce the upgrade due to lack of strategic resources. This leads to weird gambling situations, when you try, for example, to discover Ironclads, and if you do not get Coal discovered when you get the tech, you lose access to both Caravels and Ironclads and need to rely on Frigates and Privateers. Then you try to avoid Battleship tech, since it would make Frigates outdated too. Same thing happens with Gunpowder. No Niter means no musketmen nor swordsman. Avoid bombard tech at that point, if you want to keep your catapults.