You might consider getting the Total Realism Mod (2.2). One of the features in it is the Romans get all three pre-Marian units from the beginning but using Early, Middle, Late with stats improving as you upgrade the barracks. I was able to build triarii from day one. Note these units cannot be combined or retrained between Early to Middle to Late.

As for the shelf life, I agree. Mid-level units are not really built as later units arrive so quickly. Even using the TR Mod, I did not get to build any late H/P/T as the Scipii/Brutii triggered the reforms before I had a town that could. I do build peasents to move population around and get towns above 2000 and above 12,000 for the support buildings these levels provide.