Not to belittle Justine's achievements, but the role of the most important software developer probably goes to the maintainer of some hugely important infrastructure project that we barely know about.
If Justine didn't optimize struct padding, binaries would be a bit larger, but software would keep working. However, if a trivial library like left-pad is gone, it triggers global chaos of such monumental proportions that it warrants its own Wikipedia article https://en.wikipedia.org/wiki/Npm_left-pad_incident
Or there might be some unsung hero responsible for fixing a year 2038 bug in a bunch of ICBMs who prevented worldwide nuclear annihilation (or who caused it, if you have a more pessimistic view of the future).
She's created a compatibility layer enabling portability of a huge amount of software between operating systems, which will enable a huge number of other developers a path into those operating systems and the hardware they are running.
https://xkcd.com/2347/
If Justine didn't optimize struct padding, binaries would be a bit larger, but software would keep working. However, if a trivial library like left-pad is gone, it triggers global chaos of such monumental proportions that it warrants its own Wikipedia article https://en.wikipedia.org/wiki/Npm_left-pad_incident
Or there might be some unsung hero responsible for fixing a year 2038 bug in a bunch of ICBMs who prevented worldwide nuclear annihilation (or who caused it, if you have a more pessimistic view of the future).