How do you manage a 15 million LOC desktop app originating from the late 90s which contains chunks of win32 native, ATL, MFC, custom GDI+ wrappers all sorts?

Aye you fuck off and work somewhere else that's what you do. Which is why it's still written in win32 native, ATL, MFC, custom GDI+ wrappers.

They paid two companies to come in and rewrite it, first in Qt which was a complete failure. Then in Electron etc, which was also a failure.