lets do the time-walk again
The idea is far from new, and mirrors work done for IBM future system project (that eventually became the System/38 minicomputer then the AS/400), Pick, Mumps and yes WinFS.
It's also been a long time since File-systems were simply hierarchical directories for files stored on disk volumes, they've evolved into specialist databases in their own right.
If it's not new, the real questions are  what's different this time?  why do we need it?  will its performance justify the change?  is it better than other initiatives?
It's different because PostgresSQL and advanced file-systems are open-source, stable, modular and implement stable APIs - combining both functions is an engineering problem.
Why is more difficult because we no longer use raw-volumes to host high-performance transactional databases - Asynchronous IO and unbuffered file access largely eliminates FS overhead
Performance advantage of combined scheduling, memory management and indexing can only be achieved by moving the DBMS kernel into the OS kernel, but that means removing triggers/procedures or introducing a fresh attack surface for hackers
Developments around CXL Memory, pooled memory and memory semantic SSD are adding index technology to memory devices with GPGPU executing search functions - maybe the convergence of the future is database/memory not database/operating-system.