Notably, NetBSD has recently undergone a successful effort to implement such a decoupling of subsystems, while still remaining a monolithic kernel. Lo and behold, it immediately boosted the ecosystem of hobbyist FOSS OSes, by letting them easily reuse chosen subsystems (see e.g. Haiku, GenodeOS). It also certainly helped that NetBSD is BSD-licensed. If you want to learn more about the effort, search about "rump kernel".