> It's like saying if you invent the first microscope, and discover bacteria, why bother with another one?

Exactly, why bother with discovering millions of bacterias with 20 copies of the same microscope if for the same cost can build 5 different microscopes allowing discoveries of thousands of fungi, viruses, protozoans, algae, plankton etc.

In astronomy there are multiple bands to observe - gamma rays, x-rays, EUV, UV, visible light, near-IR, far-IR, short/medium/long radio waves. There are also gravitational wave observatories and special instruments like spectrographs and coronagraphs for imaging exoplanets. Every of those observations needs highly specialised instrument but brings a lot of new insight about what is happening in the universe.

Most of Hubble's groundbreaking discoveries came in first 10 years of its operation. Less in next 10 years. Even less in third decade. In fact its final service mission had been cancelled once but was reinstated once it was clear that JWST will be massively delayed and risky. For combined cost of Compton (gamma/high x-rays), Chandra (low x-rays) and Spitzer (infrared) we most likely could have built and launched another Hubble, but there was no point as those 3 generated much more valuable science than another Hubble ever would.

> But do you really believe that with one little ole' telescope pointed at the freakin' universe you're going to reach a significant point of diminishing returns?

I believe that James Webb has some unique capabilities, and a whole lot of overlap with other instruments, too. For things that you can only do in infrared, we have Herschel's history of observations, and VISTA. Sure, JWST is bigger and up at L2, and will be better overall, but VISTA's instruments have some advantages.

Not to mention all the other telescopes and ways we have of studying the universe.

There's all kinds of other things the government has spent $3B on in the intervening development time that I'd trade for another JWST. But it's a bit moot, here: NASA didn't have another $3B to spend. If you made JWST $3B more expensive, you'd not get 2 telescopes instead of 1: you'd get 0, because JWST almost died because of cost overruns.

(And even if NASA had $3B more--- there's a lot of other things that might have been better to do with it than JWSTx2).

I wonder what the optimal number of James Webb space telescopes would be? The cost per unit as a function of number built has been debated, but what about the marginal utility (benefit) ? If you built two instead of one, you would get lots of benefit from the second - an entirely independent set of scientists could decide what to do with it, and maybe explore entirely different subjects. If you made ten thousand of them though, the last one would be nearly useless, because there aren’t enough smart people to make proper use of it. My guess is that after a couple of them, the marginal utility falls off very quickly.

You might see more new things if you spend the next US$4 billion on a different instrument instead of a carbon-copy Lagrange-point infrared telescope. But you're right that you also might not.