I have played this game for over ten years.
I've never seen any realm using only infantry.
I in fact have never LED an infantry unit. I've used mostly archers since the beginning. I currently have some range 5 special forces, some range 3 mixed infantry, and have commanded cavalry numerous times in the past. But never infantry. Why? It didn't interest me, so I didn't.
The point is, that I've never seen any realm rely purely on infantry. The average I've seen is between one third and two thirds of a nation's army is infantry, the rest is archers, and then a few nobles messing around with the other possibilities.
Perdan and Astrum are two realms that partially break this. Perdan has very powerful special forces recruiting centers, even after the admins erased the MOST powerful one during the Walls and Recruiting Center Destruction Portal Stone Event. And Astrum has the best Mixed Infantry RCs in the game. So both of those realms have a higher percentage of SF or MI in their lines of battle. But even there I see plenty of standard infantry and archers walking around.
Yes, the infantry and the archers are the most basic force used by the vast majority of nobles. But there's never been any time that I've played when only one of those was used. 
Arcaea on FEI continent historically had rubbish archer centers, so while we did field archers we didn't expect much of them, and yes had to rely on massive amounts of infantry. It worked for us only because of an exceptional group of realm leaders that enabled us to have an extremely high character count.
