And not just disastrous for James Webb, disastrous for the possibility of future funding of space missions in the near future. Congress would be much more hesitant to fund anything as large as the James Webb in the future if it failed.
(In other words, how expensive would be fixing an issue and trying again actually be?)
In general it's uncommon to build a replacement satellite after an issue. Either you make a backup from the start, or you start over after failure. Making a one-off component often isn't all that much simpler just because you've already done it once a decade ago (and some parts of the JWST should be about a decade old by now). And if you commit to doing that all over again, might as well update the entire design to new capabilities and requirements.
1: https://space.stackexchange.com/questions/48902/are-there-an...