The problem is that a lot of inconsistencies are caused by the software, not I/O or cache. For example, if your database is in a middle of transaction and everything is in the memory, you can't "let the stuff complete" without installing a software on the database server to monitor power failures.
Yes, you'd have to implement this in the application layer. There would be an API which sends out the 5 minute warning to all systems, which would then need to clean up and die before hard shutdown. This has existed in the physical server and UNIX world forever.
