Definitely It will be a huge setback for astronomy and science community.
All eggs in one basket. If JWST fails, it would set back astronomy enormously. You can't do from ground what this telescope would otherwise do. –
JWST is one of the most technological advanced telescope of this decade and it is now kept safely in the L2 point and will soon start to do its work. And it is very unlikely to fail or get destroyed. Suppose if there is any software error, attempts will be made to resolve the problem from the Earth itself, BUT.....
any Hardware damage will definitely cost 10 billion dollars. Because sending tech crew for repairing the James Webb telescope to a place 1.5 million kilometer would be impractical, if done, then it will the longest distance the humanity would achieved to travel at a stretch.
And if JWST fails anytime in the future (which is unlikely) there will not be any telescopes equivalent to JWST in service for at least the next 5 to 6 years until the Nancy Grace Roman Telescope maybe....