It depends on what you mean by "open and flexible". Obviously, Windows NT isn't open source (though it has been heavily reverse engineered, and Microsoft publis

It depends on what you mean by "open and flexible". Obviously, Windows NT isn't open source (though it has been heavily reverse engineered, and Microsoft publishes kernel symbols, which is more open than most proprietary software). But Windows does have a stable driver API and a license that permits publishing proprietary kernel-mode drivers, neither of which Linux has. Linux's position is that if you want to develop proprietary drivers, either develop them in user space or sandbox them with eBPF. This is arguably less flexible than Windows's policies, though the CrowdStrike incident certainly highlights the reliability benefits of Linux's approach.