The OS is a giant collection of legacy compatibility shims which many people agree isn’t how you would design things if you were starting from a clean sheet of paper today.

If you look at software like sophisticated database kernels they bypass the OS almost completely because they can reimplement a better OS in userspace, using the kernel as little more than a device driver. Over time this has become more common, not less. OS like Linux have actively embraced this role.

Being able to effectively specialize the OS for specific applications has large benefits. It is a specialized skill but not unusually expensive to implement.