I don't disagree the Lancer is the oddball out in the entire line.
But at the point they come in the need for a cheaper front line infantry not requiring a resource goes out the window due to muskets being available.
Therefore the need to replace the pikeman with a unit that can hold the front line AND be anti-mounted isn't necessary the only thing that make the Pikes unique over the muskets at that point is the anti-mounted part so having it upgrade to a anti-mounted unit is the only major area that the next unit up needs, hence Lancer.
However I think the Lancer has a lot of inherent flaws as I mentioned before.
Maybe Lancers should be replaced by a Infantry style unit thats uses gunpowder but is specifically a Anti-mounted unit thats cheaper than muskets to build but can hold the lines like you want. But isn't as strong as Muskets but when it gets it bonus vs mounted it does do high damage to those units. IDK
I just personally feel its logical to keep 5 completely distinct Sets of units rather than have 2 sets merge and one start back up halfway.
I'm not arguing that there is no need for lancers, only that pikeman shouldn't upgrade to them.
Let say I'm playing as china and my army consists of chokonus and pikemen, which works fine as a combination. No need for expensive resource consuming longswords. Pikemen providing defence, CHK the attacking power. Clearly, I want my pikemen to upgrade to another defensive unit. But once I reach metalurgy my pikemen upgrade to lancers - clearly not the unit I want. Sure, I could build a whole load of new musketmen/riflemen, but that's annoying. I want the unit which fills the role of a front line infantry unit to upgrade to the next unit which fills the role of a front line defensive unit.
The way the game should work IMHO is to go back to how it works vanilla. The expensive and cheap infantry units merge (which is historically accurate and makes for good gameplay as there is no modern resource requirement for infantry units) and two bring back two mounted unit type:
Light cavalry: horseman->lancer->something new->helicopter
Heavy cavalry: knight->cavalry->landship->tank->modern armour
Lancers with no more moves than cavalry are useless, as cavalry are only 1 tech up and literally better at everything, even killing other cavalry. Lastly, the anti-tank gun and AA gun should be merged into a single unit as, frankly, neither are useful enough in their own right. Being good against only 1 type of unit when the enemy might yield 5 or 6 different ones is not enough. A field gun or battery (modelled on the german 88mm) would be a unit actually worth building. Similarly, the SAM batter should be replaced by an RPG unit of some sort which is good against both aircraft and tanks. The helicopter would be relegated to a light cavalry recon/hit-and-run style of attacks.
In fact, I did a mod which did the latter for vanilla, will probably re release for G&K in a few weeks, once I've really gotten to grips with modern warfare.