Well the reason that we would build the one 3 bn telescope, rather than 10 100-200 m telescopes, is because they can tell you fundamentally different things abo

Well the reason that we would build the one 3 bn telescope, rather than 10 100-200 m telescopes, is because they can tell you fundamentally different things about the Universe. In many ways, to move forward, we must go bigger with the tech, not wider. What you are essentially suggesting is a survey - we already have instruments and telescopes to do exactly that, with new ones coming online soon (see the LSST). Deep observations like Hubble was meant to do can actually be done better from the ground now (e.g. the VLT, ELT, etc.). As such, focus has shifted to those for optical observations. The community at large, however, has decided where it would like to spend its budget. That was on Webb for the IR wavebands, to study so-called 'high redshift' objects that Hubble struggles to see, and leave the more detailed imaging to those ground based observatories. Look, I would love another Hubble. That would be really useful! And that's why WFIRST exists. But an in-place replacement doesn't make sense. The tech on Hubble (not necessarily the mirror, but the _instruments_, the spectrographs, the interferometers) is really old. You can't take an off-the-shelf component, because it needs to be built to withstand space. So you need to create a one-off, 'prototype', space-hardened instrument. It is also not a case of worrying about them 'failing', but you also must worry about them slowly drifting out of a calibrated range. On the ground, you could fix the instrument, or re-calibrate it. That's not possible in space. That's not a 'random' style error, where we can get out of it by upping the number of them - they are systematic, so all 10 of your proposed telescopes would be ruined.