As another commentor said, the marginal cost of these things must be must cheaper. Why aren't they just launching new iterations like every year or so. Rather than starting from scratch after the HST, there should have been an HST 2.0, HST 3.0, a whole bunch of marginal iterations. This would create an efficient pipeline around producing many of these one-off specialized parts.
Building the mirrors would become a pipeline of incremental improvements instead of 2-decade redesigns.
I feel like if Elon Musk ran this project, he'd have built 5 different versions already, 3 of them would have failed, we would have learned something useful, and two of them would be in space delivering 2x the science.