Remember that some of the new units represent also a kind of technical progress, that can't be infinetly accelerated. Also if you want to fight against marian or even imperial units the AI romans have to become a major power by themselve, before they get them.

Epistolary Richard is preparing a late period mod submod for EB that will start in 107 BC with marian romans.
https://forums.totalwar.org/vb/showthread.php?t=51956