Instead of building one huge short life-span spacecraft, why not launch hundreds or thousands of lower cost, less capable birds incrementally? You can start with a small "first light" capable instrument with a dozen or so spacecraft, then over time and more launches put up more and more capability and decommission failed or low capability pieces.
Use some computational methods to combine the sensor collection from dozens, hundreds, or thousands of these kinds of spacecraft and you could end up with planet sized instruments pretty efficiently.
By eliminating lots of the hard engineering for the massive instruments (like serviceability, difficult to make massive mirrors, etc) you can build even better instruments that are generational and relatively inexpensive.
You get a more flexible funding story as well, e.g. fund at maintenance levels during difficult economic times and fund at larger amounts during boom times with matching launch schedules.
Tasking the fleet could be more dynamic as well, with different researchers able to reserve different percentages of a huge fleet for their specific experiments and needs -- e.g. reserve a hundred spacecraft for a long dwell "deep field" type observation, or reserve thousands for planetary imaging in a nearby solar system.
I know there are naysayers about this who think it's improbable. I urge those to think about the Starlink fleet, which went from zero birds in the air in 2015 to over 1500 spacecraft today. Now instead of radio antenna, what if they had imaging sensors and were turned to face out? SpaceX certainly didn't spend anywhere near what JWST ran as a program to reach this point and the development time has been a fraction so far.