Over the years, I have seen so many legacy systems, built on ancient technology, that I could probably demand a PhD in legacy crapware. I once worked for a FinTech company that was using Open Source components that had officially been abandoned by its developer more than a decade ago. Working with jQuery, Durandal and .Net Framework instead of .Net Core today, simply doesn't cut it. In addition, the results of working with such projects, is that you as a developer slowly withers away, and become obsolete over time. Hence, you should do everything you can, to argue in favour of upgrading these legacy systems, to a modern platform, such as Angular and .Net Core.
From your management's perspective though, this process is only costs. They know they had 20+ developers, working for 20+ years, to maintain their garbage - And hence, therefor they believe it'll take 20+ developers for 20+ years to replace it. In such a regard, your management is still living in the stone age, believing in stone age tools, and stone age processes - While the rest of the world silently gained cold fusion capacity, without them even noticing. There are simply no arguments today that dictates that it requires the same amount of resources to replace their legacy projects, as it took to create it in the first place. In fact, often replacing such systems can be done surprisingly much faster than they think, and a lot of their existing structure can also be transferred into the new codebase.