At that point, it will make more and more sense to launch space based telescopes. But... if we wait until we reach that point in the diminishing returns curve, than we lost valuable innovation time/expertise in space based technology.
My hunch is that if we wanted to get the best telescope tech in any given period of time, we should do both and watch as the two types of technologies converge.
That aspect is also much easier in space. Once you get it out there, which is hard...
In comparison, with adaptive optics ground based telescopes have a lot more leeway to be fixed and corrected.
(That said, if astronomical interferometry can achieve the same result as a 100Metre telescope, then let's do it that way! How is less important than making sure we have something in our pipeline within the next decade that can be capable of directly measuring the spectra of exoplanets.)
https://medium.com/the-physics-arxiv-blog/how-to-build-a-qua...
https://m.youtube.com/watch?v=v6ihVeEoUdo
It has a 6.4m (21ft 4in) primary mirror. Which, from a pragmatics standpoint, answers your question.
Though I suspect mission plans could be drafted for a larger scope yet.
Despite it's impressive number of steps, this does not look like good engineering.
Instead, it looks like a design by committee, with engineers going drunk on complex parts coming together rather than dumbing down the unfurling of this telescope down to its most basic possible mechanism, with the fewest moving parts possible (where failure is chronically always possible).
Seeing how overcomplicated this is, it's no surprise it cost $10 billion to build.
You're looking at a high-precision instrument, that's being fit inside a distinctly finite space and mass budget, that will have to operate for years (possibly decades), and which is beyond reach of any conceivable service mission for a very long time.
Movement, thermal control (the Webb operates in the low end of the visible spectrum, through the infrared, so its temperature itself is critical), alignment, and drift, all matter.
And every piece of equipment has to be tested and trialed to the maximum extent possible.
Yes, it's a bit Rube-Goldbergish, but then, so is the specification.
I wonder if you could make one with a few bits of lens that fold out? They'd have to be positioned more accurately that one wavelength of light (to ~100nm say or a 10,000th of a mm) which is tricky on earth but might be doable in space where you don't have gravity making things flop?
Space-based telescopes are a better avenue. You can achieve much better resolution in practice with a much smaller telescope. There's no atmosphere to block you, and instruments can capture light of any frequency. Cost is the main disadvantage, but delivery of satellites (and maintenance) to space is getting cheaper and cheaper.
All of which is to say that I agree we should be spending more on astronomy. That was your main point, of course. I disagree with the particulars, and think we should be more willing to go to space.