> There's a fair bit of evidence that things we've traditionally assumed belong in the kernel actually belong in userspace, and they're being reinvented in userspace as a result.

That's basically the pitch given by unikernels like Mirage: functionality which was traditionally implemented by an OS, like storage, is turned into a library that gets compiled into the application like any other. If an application wants to access a filesystem on the disk it can use an appropriate library; if instead it wants to manage the data being stored on (some section of) the disk directly, it just needs a different library. That way, applications like relational databases can claim their own section of the disk and read/write it directly, to avoid the performance and reliability (e.g. caching/flushing) penalties of going via a filesystem.