One unified OS for the future?

[Minuous]

4GB RAM no longer cuts it.

4Gb is a huge amount of memory and should be enough for any conceivable purpose, if the OS and application aren't poorly written bloatware.

a cooperative multitasked OS with incomplete preemptive scheduling, no network stack out of the box and severely limited security and performance

Are you talking about any of the AmigaOSes here? Because they all have full preemptive multitasking and TCP/IP stacks, and they all are very efficient (especially OS3.9) and thus highly performant.

[Minuous]

Uhh guys I'm not redefining any terminology: preemptive multitasking by definition requires the OS have the ability to cull tasks that overrun other tasks, or hijack the scheduler, as one can effectively do currently with the forbid() system call.

No, it doesn't require that at all; preemptive multitasking is about task scheduling, nothing to do with memory protection. See eg. http://en.wikipedia.org/wiki/Preemption_(computing)