I custom compiled a kernel to support userland chroot a number of years ago; it was trivial. It served the use case just fine. There are probably people today

I custom compiled a kernel to support userland chroot a number of years ago; it was trivial. It served the use case just fine. There are probably people today who run things in Docker containers as root who are thinking "OMG, this is a horrible idea!". In the chroot environments I created at the time everything was statically linked and there was nothing else for the program(s) to run. Everything was audited. It didn't start with "it works on my machine, so why can't I ship my machine?" as a premise.