I don't know, I notice pretty much the opposite in the field of software I know - smaller software come and go, but the large ones were there 20 years ago and will likely still be there in twenty years
The maintenance of giants is tied to their importance rather than the complexity of their codebases. SQLite is no less likely to be around in the future because it is simple.
Codebase complexity also isn't just about size. A huge, well-written codebase is still more likely to stick around in its current form without requiring a total overhaul.
There is a lot of inertia driving maintenance on some of these large products but there is also a ton of deprecation taking place internally relative to them.
Obviously, if you're writing a simple CRUD application, you'll never need to go down there. (You'll probably make a browser application or an Electron application.)
But if you're doing something unique, like a utility, or trying to hook into the OS to make it do something new, you gotta work at the lower level APIs.
And, yes, I shipped a product that included a Windows driver this year. It required working at the Win32 level in user-mode to communicate with the driver, and to test the driver. Windows recently added a new API to Windows 10 that eliminated the need for a driver, but we still had customers on Windows 7. Even then, that new API is technically Win32.
I can list 10 billionaires in the world, but being a billionaires is not normal...