Well, to be fair, historically armies do seem to have evolved more dramatically over the Medieval period than in that of republican Rome. Play any of the major RTW realism mods and there is precious little of a "tech tree" to speak of, with regard to units. Take the Romans for example - basically, their core army becomes all principes. Rather boring - brutally effective, no doubt, but still boring. Moreover, the realism mods tend to make it available to recruit from the full unit roster right from the beginning (waiting decades for triarii does not make much historical sense).

Some of the vanilla RTW factions have pretty impressive tech trees in terms of units - the Seleucids in particular. They start off with levy pikemen and prodomoi and slowly tech up. I think I was at the gates of Rome before I could get imitation legions. Never saw an armoured elephant (or even a Companion, IIRC).