Matrix, just check my 1912 Rifleboy and you'll see it is far from obsolete where it sits:
- Hill, forest, fort, fortify bonus = +125%
- Woodsman III, Guerilla III = +100% and 2 first strikes
- Drill IV = 6 first strikes
So base = 14, + 225% defense = 45,5. Before it is being hit the 6 first strikes do a nice job.
Then at level 15 it became an additional:
- Combat II = +20%
- Ambush, Pich and formation = +25% vs. a lot of units
Which means an attacking Modern Armor is first hit by 6 * (14 + 45%) = 20.3, then has to fight against a defense of 45,5.
I doubt you can promote a Spearman to a level where it can stop a Tank btw but you could give it a try.