Fwiw, if performance isn't paramount, you can have a lot of fun with LKL (linux kernel library) and UML (user mode Linux).

UML is full Linux running as a userspace process. Tools like guestfish use it to modify disk images. UML can only run on Linux though.

LKL lets you use the linux kernel as a library. However, only single threaded. But this allows you to use linux's tcp stack from userspace, or its filesystems, etc. LKL also works on non Linux platforms.

These are definitely a different shape than what you're looking for, but they could help you keep the real linux implementations alive from userspace far into the future.