All it takes is four military technologies in a row to unlock post-Marian reform legionnaires and this can be done by 260 BC. Why would CA do this? Of course I could "hold off" on researching that technology, but I would be severely limiting myself. This is simply poor design. I suspect CA chose to ignore ~160 years of Roman military organization under the manipular system in order to rush out the "cooler" and more iconic legionnaires. I am usually forgiving and try to mentally justify other immersion breaking design choices but this is absurd.
Additionally, the assault trireme general that you have at the start of the campaign has the model of a legionnaire that you can use in battle. Why would CA not even bother to give the trireme general a Polybian model? I am clueless.
Ideally I would like to have the technology structured so that half of the game would be under the manipular model and the other half under the Marian model but I suppose that is what mods are for.