The Hubble Space Telescope has been repaired more than once, and after decades of operation, it is still booked solid for future science observations. There is no forecasted date when it will no longer be needed.

Even if the James Webb Space Telescope is successful in the short term, it will stop functioning in a few years, and that will be it, because there will be no way to repair it. I would NEVER have designed such an obvious flaw. Maintainability is THE PINNACLE of refinement in technological development. If something is not maintainable, they didn't spend enough time on the design, and it is more than likely a piece of garbage (or will be at the first hint of a problem).