That makes me think - Astronomy beeline doesn't work with TC, btw.
A fair point. However, I think that if we beeline Metal Casting, say via The Oracle, particularly if we have access to Copper, then we'll give The Colossus a long enough lifetime.
I seem to recall Gosha advocating building The Colossus even in a game where he got Astronomy early on.
While you can look this issue as Astronomy disabling The Colossus, I think that a better way to look at it is that if we can build The Colossus, it can pay for itself VERY quickly. It costs 375 base Hammers. With the required Forge and with Copper (we can probably skip building it if we lack Copper), then the actual cost is 375 / 2.25 = 167 Hammers.
Let's say that we are working 3 Clam squares and even, say, only another 3 Coastal squares total. That's +6 Commerce per turn, which comes out net-neutral after 28 turns... although "Hammers now" or "Commerce now" is of more valuable, so let's say that it takes, I don't know, 35 turns to fully pay for itself, were we to otherwise spend those Hammers on Wealth.
If we ignore the Great People Points (which we will probably want to ignore if we are going to try and generate Great Scientists... meaning building it in an auxilliary City), then we just need to ensure that we'll use it for more than 35 turns on Epic... not too hard to believe.
So, assuming that we take longer than the number of turns to build The Colossus (say, in a base 8 Hammer City, that's 167 / 8 = 21 turns--and a lot less if we 2-pop-whip other build items and overflow some Hammers into the Wonder) plus 35 turns = 56 turns or less on Epic to research or trade for Code of Laws, Calendar, Compass, Optics, Machinery, and other techs that we might want like Currency and Alphabet, then we should still target building The Colossus.
However, the location of WHERE to build The Colossus (as well as The Oracle) becomes an important issue.
Should we agree to aggressively a Lightbulbing strategy, accidentally getting a non-Great-Scientist for one of our 3 (or possibly 4 if we build an Academy) Great People is going to be incredibly costly.
So, can we afford to sacrifice one of our two on-continent Cities as a non-Great-Scientist-making City if we settle in place, since we'd put The Oracle in one of these Cities?
Is it even possible to compete on building The Oracle in a 3rd settled City?
Would a good compromise be building our second City off-continent, if we do settle in-place, so that we could still have two on-continent Cities that are dedicated to generating our last 2 Great Scientists without having any Great People pollution? If we do so, is it possible to get an off-continent City (settled as our second City) built-up quickly enough in order to compete on completing The Oracle?