My understanding is that the JWST opens up new observation spaces, specifically very distant and highly redshifted objects that Hubble couldn't capture. So we should have a lot of data on a new class of objects in fairly short order, and thanks to the cosmological principle we can expect to see similar distributions of the same objects and phenomena no matter what direction we look in. As we gather more data, we will converge on an understanding of these new spaces, and eventually the error bars will shrink to the point where further observation is generally not giving us much new information.

Is it better to build more JWSTs, to accelerate that convergence by low integer multiples and similarly increase the chance that we'll happen to point one at something truly new and "surprising"? Or should we spend our money on bigger and more capable instruments that we know will give us access to entirely new observation spaces that are completely out of reach of the JWST and other extant instruments?

I don't think it would be such an obvious question, if we weren't constantly getting better and better at designing and launching large and complex instruments. As things stand, we can let JWST and its ilk blaze the trail, then follow up with cheaper instruments building on lessons learned and our general technological and economic progress.

Depends. Is experimental time at the LHC anywhere near as competitively-rationed as it is for a space telescope like Hubble or Webb?

In fact, answering that question should come down to whether we expect the LHC to realize the majority of important observations "in range" of its instruments within its projected lifetime, and also in time to make useful contributions to the selection and design of future experiments.

To answer your question directly: yes, AFAIK the LHC is fully utilized; when it's not running, it's down for maintenance or upgrades, and its observations are (or have been, in its active periods) in high demand. But it's far from clear that building another one would yield a good return on investment, especially given the opportunity cost of diverting funding from future experiments.

I'd much rather broaden our view than double it in one narrow band. (Or advance fusion research.)

Mathematically, you are quite correct.

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?

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.

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).