Those 20 years weren't about current functionality, but tons of changes reflecting how business requirements were changing. New system from the scratch in the s

Those 20 years weren't about current functionality, but tons of changes reflecting how business requirements were changing. New system from the scratch in the same technology would be very cheap (given there are folks around who still know original language/platform). System being completely rewritten - I can't agree on that one either. Some, in some companies do, most don't. I personally manage quite a few of those and there isn't budget for costly migration experiments if things work. Working in one of the biggest global banks in Switzerland. What works is if ie OS/browser that is absolutely required isn't supported anymore and there is no way around it. There are very few cases where the approach 'if it isn't broken don't fix it' isn't the best approach for the company. But good luck trying to explain it to devs/teams who want to try new things (basically at the cost of employer, I don't blame them but it is what it is).