[keeperofdakeys]

Probably never. ZFS isn't just a filesystem, it was developed to be an entire storage system that's vertically integrated, so ARC is a fundamental part of the filesystem design.

ZFS also has a huge legacy. Right now the license (probably) prevents you from legally shipping a compiled zfs module with the linux kernel, just solving that seems insurmountable. It's also supported on Illumos and FreeBSD, trying to refactor it to use the linux page cache would have a chance of introducing bugs to these platforms.