One argument for using expensive, powerful units is that if you have plenty of resources, they are a more efficient use of the required army population (at least 100).
However, cheaper “trash” units require fewer villagers to keep them going, so it’s possible to sustain a lower population below 100.
In one game, I was making halberdiers from nine barracks with conscription and found that I had too many lumberjacks and farmers (40) to sustain them.
It could be argued that I could have dropped to 70 or even 60 population and still maintained production.
I had all upgrades and only used gold for the occasional siege unit that I exchanged at the market.