You don't have to rewrite all software every 3 years. After 5 years without any major changes the software is stable, it has been battle tested, all issues and

You don't have to rewrite all software every 3 years. After 5 years without any major changes the software is stable, it has been battle tested, all issues and quirks are well known, features and designs has been discussed. It's now time for the software to pay off the development cost. And for a project like this - it's probably saves millions of man-hours every year, time that can be used for other things.