Have you read the entire thread you linked? People explained to Linus why direct IO is important to them. Besides, it's 20 years old and there were even no SSDs back then. The lack of direct IO in PG was one of (one of) the reasons why Uber moved to MySQL (https://www.uber.com/en-PL/blog/postgres-to-mysql-migration/, "The Buffer Pool" Section). With buffered IO you will likely store a lot of the same data in memory twice- once in DB's memory and then in page cache. Now you can just give the memory used by page cache directly to DB, because it knows better what and when it needs.