I am no longer a junior, but would have been upset to be tasked with refreshing the old historical obsolete laundry (no matter how sacred or distinguished), expecially when I already had experience delivering safety critical products packing much more modern technologies.
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.
What have you worked on that is as cool as a space probe that's cruising in interstellar space and still collecting valuable data?
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.
Is that because juniors want to leave their name on something? I ask honestly since I shared a lot of the same sentiment as you, and never quite got an understanding as to why working on the cool new thing was "more fun" even if a lot of the projects under-the-hood were recycled.
Also, many decisions taken Probably can be traced to limitations / idiosincracies of the era
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