Interesting comments. If I understand correctly, we agree that without -metals forges are not good. There are a few things I don't understand. Why do you say "2 extra pop by not building the forge"? We know it's a 4-pop whip. You can 2-pop whip HAs (intending to upgrade) and let HR take care of happiness. Cold fact is what I said earlier - you need to whip 16 pop for a forge to break even -wise. We definitely agree with the snowball effect, but I think it's to the opposite direction.Cities with high food surplus should get them, slow cities are better off without them, naturally, but for me it is not just hammer reduction the forges give but also the burst mode they allow you if you have any of the happy metals. Even if you start with 2 extra pop by not building the forge, you end up in a problem very quickly as you need to grow into unhappiness if you are whipping hard (as you should). Needing to grow to +2 or +3 unhappy people slows down growth and whipping additionaly. And then you face a bit extra resistance and lose more units, which slows you even further. There is a negative snowball at play here. Attacking or reinforcing 1-2T slower can totally change the course of the war.
One explanation is that you didn't have upgrades in mind, rather go with no GMs in which case forges make much more sense.