shakabrade
Praise Vivec!
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.
One explanation is that you didn't have upgrades in mind, rather go with no GMs in which case forges make much more sense.
Yup, no happy, no good. Unless IND. You can play around the need for Forges, I also like to use GM and upgrade (WEs more favored than HAs). But you need a nice map/traits to make all that work.
I say 2 pop more without the forge because you can't usually grow 4 pop more due to happy cap, each level costing more food and because you will have some OF put into forge. So usually it will not result in 4 more pop, plus it is a good chance that your forge is in the Mono city or Bureau capital, reducing the pop lost to whipping. I did no statistics on this but my feeling is that Forge costs around 2 pop and some lost commerce due to not working as many tiles. Maybe because I do not consider building it if it is a worse deal. Survivorship bias.

Regarding the 16 pop to break even, it is a worst case scenario, because of opportunities to reduce the Forge cost. Extra AP hammers can also play a role. If one is going for Forges, one should also play around them. Not just build them because it is a thing to do. So, what ideally happens is that you either have forges and love them, or don't have them because they have no business case. But I also feel that forges are situational and not as good as most of the people think.