However, for performance reasons, they do not exploit the isolation features of the hardware. All application code runs in a single address space and at the same level of privilege. While this strategy is inherently dangerous, there are compensating controls imposed on application code. The strategy has been very successful. The res systems have achieved extraordinary reliability and stability for any kind of system, let alone systems which are both mammoth and monolithic.
Portability is another issue.
Like most application code, the code of the res systems is sensitive to its environment. It expects a certain application program interface. The API isolates it from the underlying hardware and operating system. The application code has survived many ports to new hardware and operating system. However, the application is so large and complex that it is difficult to view it as anything but a monolith.