You're right about Bell-LaPadula, but note that model isn't used as much as the Biba model, the basis of Windows UAP. Even "solved" problems tend to evolve or get superceded over time; nothing seems to stay solved for long!
Windows UAP is a horrible thing. It would be far better to just replace the system dialog boxes like file open, save, etc. with power boxes that then give file capabilities to applications.
It would be minimal work to refactor applications, and provide almost perfect security with no UX change.
This is how macOS apps work by default these days. Each sandboxed app can R/W to a private "container" directory only, and to access anything else you present an "open" dialog that gives you back a special URL object with the requested capabilities for that resource. This is pretty fascinating: https://www.mothersruin.com/software/Archaeology/reverse/boo...
