BSD actually has this already.
I'm not sure how it'd defeat the point of having their own kernel.
As for cost, possibly, but it would really be a huge boon to macOS for software devs. It's hard for me to believe that Rosetta isn't similarly costly, but it's been done because running x86 software is still very much a necessity for MacOS.
Because then you'd need to both maintain your kernel AND your own implementation of the Linux ABI, an ABI you don't have control over and that basically forces you to reimplement half on Linux in the first place.
People already get what they want by having a tiny Linux machine running at native speed. In 2026, virtualisation still isn't free, but it's pretty darn close.
A very large portion of that ABI is already implemented due to both systems being POSIX. But further, a lot of what programs actually interact with is already ported to macOS. For example, you can build and use glibc.
Also, I get the lack of control, but that really isn't a major issue. The linux kernel pretty rarely adds new userspace additions. By and large the majority of work that goes into the kernel is around new drivers and fixing drivers. Even when there's kernel level features, it's very often not a userspace thing but rather things like new schedulers.
There's a reason MS didn't see the same approach as being too terribly crazy with WSL1, and those are very different systems. Heck, there's a reason cygwin continues to exist and work.
POSIX provides an API, not an ABI, and that API kind of ends at libc. Being compatible with Linux at an ABI level means being able to provide the same syscalls in the same way as Linux does. Not all Linux syscalls map cleanly to POSIX APIs, and in general xnu has lots of different concepts that make it somewhat cumbersome to adapt to what the Linux kernel does. The example of this is Microsoft with WSL1; they gave up not because Windows was too shoddy but rather because people want ALL of the kernel, which is a moving target anyway. it's a waste of time not to simply run it in the first place, virtualization is cheap and you get the real thing, with no quirks
There's also simply the possibility of using linux software directly in macos without doing OS dependent changes to the software.
Running VMs is really really easy and low maintenance demand on Apple. And it’s guaranteed compatibility.
Wasn’t compatibility what really sunk WSL1?
Yes, but a big part of the problem with WSL1 was the size of the conceptual gap between POSIX and Windows NT that WSL1 had to bridge. An “MSL1” would likely have fewer problems because the gap between macOS and Linux is smaller, given they are both POSIX
The other thing Apple could potentially do, is add Linux-compatible APIs to macOS. IBM wanted to support Kubernetes on their z/OS mainframe operating system, so they implemented on it a clone of Linux namespace APIs, e.g. unshare. Then we could have macOS nodes in a K8S cluster-which might actually be useful for some people, e.g. if you have a Jenkins CI farm, the Linux nodes can run on K8S, but currently macOS nodes (which you need if you are targeting iOS or macOS) can’t, they have to be bare metal or VMs.
More Linux-macOS source compatibility would also benefit macOS by making it less work to port software to it from Linux
It's true that missing ABIs will cause random crashes and problems. However, a lot of apps can run with a minimal set of ABIs.
The target for this isn't GUI stuff.
Maybe that’s not what they intended.