It depends on what you mean by "multi-decade old app".
Excel is a multi-decade old app. Windows is a multi-decade old app.
They're both still being actively developed with a lot of new and cool features added though, and that keeps it fresh.
The multi-decade old app that hasn't been touched in 20 years? It need a rewrite. And yesterday.
I suspect though, that is not what you meant.
And we need to stop all new feature work on Excel since its legacy, so give me 80% of the dev team to do above. Oh btw, they don't know the alphabet soup of stuff I decided to use, so we will also start firing them as well, as I need to hire for these special skills.
Eventually styles will change and you will have to redo the UI. This will happen much more often than the above. Your program may look very different but if you have a good architecture this is a superficial change. It may still be expensive, but none of your core logic changes. Normally you keep the old and new UI running side by side (depending on the type of program may be different builds, other times it is just a front end) until you trust the new one. (depending on details it may be an all at once switch or one screen/widget at a time)
https://www.rocketsoftware.com/en-us/products/cobol/visual-c...
Anyway, as soon as LLM can reliably produce executables directly, languages will lose their value, it is the new Assembly to high level evolution.
What does that suppose to mean
To frame your argument, TIOBE ranks COBOL as the 19th most popular language, ahead of Ruby.
It's beyond useless.
This is a better list, and cobol is not even part of the top 20, while Ruby is at 9th place: https://redmonk.com/sogrady/2024/03/08/language-rankings-1-2...
It's like I'm sitting in a meeting.