tomscharbach I would imagine the cost to replace in this instance would be in the hundreds of millions (writing, testing, security,implementation, etc.). Edit- although thinking about it now, that would be the cost for a drop in replacement. You could probably do it for far cheaper, breaking it down into component pieces, using off the shelf software that met specification as you can, and only write from scratch only what is absolutely needed. Is that type of fragmentation (or maybe modularity) better or worse in the long run? Dunno.
No one is even going to broach that until it is painfully clear the cost of not upgrading is more than keeping it around, or blame can be shifted to someone else.
Although they may have their hand forced with W11 and half-ass a buggy mess while they bicker for 5 years about what the new implementation should be (judging by the rollout of other bespoke software in use).