The Apollo was actually fairly decent for its heyday (2326-2334) and served fairly well against both the PVN and the Shivans. Its strong hull and easily refittable chassis served it well during the chaotic years of the Reconstruction, too.
Plus, I also think the GTA used the Apollo in some form for assault and bombing missions prior to the Herc and Medusa. That's right, the Apollo chassis. Modified variants, yeah, but why come up with a new design when you can churn out lots of parts for the old one and adapt it to a lot of roles at once?
Granted an Apollo isn't going to do as well as a shiny new Myrmidon (which among other things is faster and more agile) but hey, it's not 2335 anymore, so what do you expect?
Plus the GTA was pretty cash-strapped in the Reconstruction, as evidenced by their inability to stay in one piece, and their massive industrial base in Sol. Thus it was unlikely that the development of the Myrmidon really crystallized until some time after the GTVA popped into existence around '45. Apollos were probably used in the boonies and myriad Terran Blocs and GTVA for a while (well into the Fifties is my guess) and even longer as training fighters.
So, in short, the Herc may have won the Great War, but the Apollo was the stalwart of both the T-V War and the Reconstruction.
*stands firmly behind Apollo*