9/10:
UNIX was a successful system, whereas Hydra was not. Given that you've read both papers, what is the best
technical
reason for this?
The success of UNIX compared to HYDRA is due to the fact that while HYDRA's complex resource and rights machanisms allowed more flexibility and expandibility for system designers, UNIX's focus on simplicity meant that a complete and usable environment could be finished while the HYDRA team was still working on implementing subsystems. As they acknowledge, "The existence of a kernel ... does not make a system usable." [Wulf 1974] Instead of trying to create an environment in which anything is possible, UNIX created an environment whose simplicity and elegance made use simpler while still powerful.
In the UNIX philosophy, smaller programs do their own particular jobs very well, and these programs can be strung together in just a few different ways (i.e. pipes and redirects) in order to accomplish more complex tasks. In HYDRA, interactions between different resources depended on programs knowing about the capabilities of any resources with which they wish to interact, and how to properly use those capabilities. Thus while HYDRA hoped to be flexible and expandable, this came at a cost of complexity, since programs had to be constructed with a knowledge of how they could interact (which, ironically, limited future flexibility when new object types were created about which current programs were unaware). In UNIX, everything from regular files, directories, to devices were simply treated as files. HYDRA, on the other hand, envisioned an infinite variety of types to handle all the possibilities withint these different situations. UNIX had a powerful shell, which was flexible but also provided a common base for all users. HYDRA made no such provision, but imagined individualized user environments specifically tailored to their usage.
UNIX aimed for "simplicity, elegance, and ease of use." [Ritchie 1974] HYDRA provided more extensibility than desired, requiring too much effort in order to create a usable system on top of the kernel. Thus, UNIX succeeded in the marketplace because it provided a complete kernel plus user system for a common machine.