The reason why saving Hubble is such a big deal is that NASA rarely
ever carries out most new programs through flight status. So, if HST is
not saved, its possible that replacement would be cancelled, leaving
the science community with nothing.

Your comment is indicative of precisely why new projects are often
cancelled before flight - let's say that someone proposes the New Great
Telescope, and NASA funds the program. After a couple of years,
technology advances and the New Great Telescope is now just the Just OK
Telescope, albeit at the same price as before, if not higher. So, the
Better becomes the Enemy of the Good Enough, the new telescope is
cancelled in favor of the Next Really Great Telescope, and so on.

Better to hold onto existing hardware until the replacement is on
orbit. So, we really need that robot to save Hubble!