Yeah, we cannot see very far into the future and betting that system software will remain stagnate is not a good bet. If that means wholesale replacement, few of us will understand it at first. Otherwise retrofits will remain the norm for example one old idea that seems to be on the up and up is hardware-enforced capability systems and those working on them have found plausible ways to retain most of the existing C ABI world. Whether it always remains a good idea to continue propping up the past is one of the hardest questions and ultimately will be more of an economic one.

There are obsolete systems that meet your criteria, Plan 9 and Apollo Domain/OS had plausible early answers for networked computing that haven't been adequately explored.

For your last point, I would say scheduling, security, namespacing are the core features and everything else can be layered (perhaps interchangeably) on top. As a practical example, even in monolithic FreeBSD it supports fully runtime swappable TCP/IP stacks and out of tree network stacks.. the core illusion for that system is retaining the same Socket interface for userspace. Presumably if you get the namespacing thing right the entire system and network of things would look a lot more like deliberately interchangeable parts.