Sure, sure. Everything is always situational. But one thing is sure: If I'm Inca, I'm not going to build a single monument in the entire game.
What I on general level as Inca will try to do is to make sure that I have at least one food resource in the first ring or that I can borrow one from a nearby city. Then I try to get 30 hammers in (maybe with one chop) before my city has more than half of it's food box full at size 2, and then 1-pop whip the Terrace. But again it's situational. If I have Maths and the city really requires a border pop, I would try to organize my micro so that 2 chops come in on the turn the city is founded.
I just wrote a lengthy post about the benefits of earlier granary in the best/worst leader trait - thread. The same principle applies here. If you have to build a monument before granary, it sets you significantly back.
But yeah, as you wrote, we probably don't disagree about the importance of early granary. What we are probably disagreeing is the economic value of SA. Sure it is one of the more useful UBs, but as I said, your pop points still bring exactly as many hammers as before. I agree that it has value! Sure! It makes playing easier as you don't have to pay some much attention to whip unhappiness. And it definitely makes life easier in cities with lots and lots of food, but no production.
But the main thing here is that SA's benefits come so much later than Terrace's that a competent player with early bonus has already snowballed into the significant lead. (And the other thing is that I don't believe that the actual hammer bonus that SA's create compared to normal 2/3-pop
is really that big. Or can you quantify it somehow?)