Though obviously in the very initial stages, the implications of this development taken to its end could be enormous. Have the GNU Hurd implement translator or server interfaces to the rump drivers, plug it on top of an immutable package and configuration management suite like Guix (actually already being done), and for the first time in history you have a complete, general-purpose microkernel-based Unix-compatible (but beyond plain Unix) system that could rival the likes of GNU/Linux. The great thing about it is that the Hurd isn't afraid to break POSIX semantics where beneficial. This makes it have, among other things, processes running under multiple uids, true Plan 9-style namespacing, unprivileged mounts, token-based authentication, and so forth.
I've been playing with it on-and-off (including cross-toolchains) and been pondering about Mach kernel emulation on Linux. A pure userland server would be most desirable, but faces real changes with modeling port rights as file descriptors sent over ancillary data, how to reliably handle user-level page fault handling on a per-application basis, mapping VM regions across address space boundaries and properly emulating RPC. Kip Macy did successful work on a port of OSF Mach from the MkLinux sources as a FreeBSD kernel module for use with launchd, XPC and other OS X modules, so I think that might end up being a concession I'll look into taking one day. I'm occupied with other projects in the meantime.
It'd be also funny to have Linux "eat" itself in such a manner by becoming a host for the Hurd. Poetic justice, I suppose. But I'll wait to see if the rump kernel stuff pans out first.
(MINIX 3 is great too, much better microkernel, but it's more meant for reliability rather than exposing end-user flexibility like Hurd does with translators. It also doesn't come close to the ~81% of Debian building as is with Hurd, and the MINIX devs themselves are positioning it as an embedded platform rather than general-purpose.)