The issue, IIRC, for JWST is that it's IR optics need to be cryogenically cooled, and even in the vacuum of space that coolant will gradually boil away. Hence w

The issue, IIRC, for JWST is that it's IR optics need to be cryogenically cooled, and even in the vacuum of space that coolant will gradually boil away. Hence why JWST will have a shorter lifetime than Hubble (L2 orbit, station keeping, and serviceability notwithstanding).