Even if you can afford the best troops, you still can't just throw them away, as the population mechanics will punish you for doing that. For example, I have ran out of foreigners to conscript for my auxiliary troops in my capital, and now I must journey further afield to find auxiliaries, and they won't be the same types that are available in Antioch. I have also ran out of Babylonians to form into heavy spear infantry in Seleukia and Charax, and additionally only have enough elites to form one or two elite cavalry units with in those cities.
I was flush with cash when I incorporated the elephants, then diplomacy took a turn for the worse as I turned 'unreliable' and a bunch of randoms cancelled their trade with me. I think it's a really stupid mechanic that they would cancel a good thing, but there it is. Now my income is half what it was ten turns ago and probably still on a downward tack.
I guess the point is you have to actually pay attention to the economy and cannot simply field the best of everything all the time.