It's worth considering that nowadays we don't really do backup copies of satellites/probes because launch is pretty reliable by now, especially on rockets that

It's worth considering that nowadays we don't really do backup copies of satellites/probes because launch is pretty reliable by now, especially on rockets that have been flying for a while (which is pretty much a requirement to be chosen to launch a flagship project like JWST). Ariane 5 has a long flight record with very few failures and the design has been further carefully reviewed specifically for JWST, with the previous two launches having served as verification for any fixes that needed to be made prior to flying this mission. Basically, the chance of JWST being lost due to a rocket issue is much lower compared to the chance of it being lost due to design issues with the telescope itself.