I've seen projects akin to "Excel sucks because its written in X, I am very smart, we will rewrite it to run in browser only with this new framework I read about, backed by microservices in Kotlin/Rust/some new fad, running server less on AWS (using this new thing thats just out of beta that I saw at re:Invent)".
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 you need to admit COBOL is dead and rewrite. Eventually some library/framework is dead and you need to rewrite. However there are lots of options, and often your best is in place rewrite just small parts at a time, slowly getting rid of the legacy code over a couple decades - there is no real hurry here.
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)
Just because it is populate doesn't mean it isn't dead. Or should be dead. COBOL was really innovated in the day, but many of those innovations proved to be bad ideas. However switching to something else is very hard and expensive - thus it continues on.
Well, I'm not sure I'd say Windows is a good example of that anymore, in fact I was going to use it to argue the very point: as they seem to rewrite the UI in new frameworks, we've lost a lot of features (not even talking about speed and reliability)
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.