First post in balance forums here. Completely disagree with that assertion. I think RTSs should at least attempt to provide useful roles for units through all stages of the game. Otherwise you're punished for having good unit preservation by being stuck with a bunch of useless units sucking up your population cap. Which is silly.
A good example of a unit that was given great scaling was the Sd.Kfz 251 halftrack in CoH1. After it ceased to be useful as a frontline combat unit, it could be rigged up with Wurfrahmen 40s and used as an artillery vehicle. Which was great because it meant you didn't have to suicide your vehicle for pop-cap after heavy vehicles hit the field.
Thats not really how RTS games work.
Even in best RTS games, early game units are being replaced by late game tech, look at one of my all time fav RTS-WC3, footmen replaced by knights, orcs by taurens and so on.
Even in SC2 you wouldn't be using marines and zerglings after certain time, would you?
Its no different here except for the fact that one army is absolutely forced to use units that become completely irrelevant late game.
If you preserved your AGs, just use them for capping, they'll beat all other capping squads effortlessly.
And your example is flawed(just like it was flawed in coh1), you had late game unit with a price of early game one-if that is not the definition of imbalance, then I don't know what it.