mmap(2) in Unix predates Linux itself by several years (1988 Sun, 1990 BSD). mmap(2) was present in Linux well before any of the high performance databases, file systems and other applications that use(d) O_DIRECT appeared on Linux.

O_DIRECT was resisted by Torvalds and others, but it's there today, and used by supposedly important platforms. Nvidia even has an API to DMA data to and from GPUs that uses O_DIRECT. There are folks working on new async WAL O_DIRECT for PostgreSQL as well.

So what can be said? mmap(2) isn't sufficient for certain cases. Torvalds has faced and successfully navigated difficult design choices in a pragmatic manner. What else?

The lesson I take is this: there are levels to implementation complexity and value. The wisdom to decide the appropriate abstraction level is crucial. You are unlikely to be equipped well enough to beneficially utilize O_DIRECT, but it's not impossible and some, in fact, are so equipped.