Infantry are more reliable especially when you have as much high quality infantry as Aurvandil is. An archer army may get lucky and beat them in one battle but it's a flip of the coin whether they are any good or not. I think, unless someone get's just an unbelievable cav center or two that very few other compositions could beat the infantry mass. Look what the Romans did with elite infantry and all their other units being pretty mediocre.
I'd also tend to say that the bits of cavalry and archers are much more effective in an infantry-heavy army than in any other composition.
Archers can get a few good shots, but they get totally pwnt once in melee.
