The opportunity they would be offering is not rare at all! The opportunity to research and design something truly new on the other way is very scarce.
And you're left with a codebase that has been in hands of 6 Decades of probably great engineers that have already done a lot, plus any of the arcane cruft of such a long lived and esoteric project
It's a great CV highlight, but I don't know if it's the best learning opportunity
There are a lot of things as cool as, done by people I know, such as the gyros on the Webb telescope, the APU in the F-35, or a small rack-mountable Cesium reference clock, but there aren't many opportunities like that.
I won't write about the projects I've been involved with for privacy, but to give you an idea some of my old team members were involved in ams-02 for example.
> We’re programmers.
> Programmers are, in their hearts, architects, and the first thing they want to do when they get to a site is to bulldoze the place flat and build something grand.
> We’re not excited by incremental renovation: tinkering, improving, planting flower beds.
The opportunity to maintain software running on a spacecraft is not rare? I don't think so. And those two particular spacecrafts? I'd take that job in a flash.