|laziness, impatience, and hubris|
the features that eventually became attractive enough to be "must-have" s in other systems, were built-in right from the start in *nix.
Hm. I've been trying to avoid getting into a full chronological review of OS development as it is all pretty well documented on-line, but the reality is that if modern users of (say) Linux sat down at the console of a machine running the early version of unix, they would find very little that they recognised.
And if they attempted to program those early versions, they'd feel that same way that modern kids would if you made them swap their iPhones for CB radios. Very little of what constitutes a modern *nix, even by the original POSIX-1 definition, was a part of unix "from the very start".
Even such stalwarts of what is now seen as fundamental *nix--tcp/ip, sockets, locking and much more--simply didn't exist until many years later after the unix-wars, when the IEEE POSIX initiative pulled them back together from System V, BSD, Xenix/SCO etc.
And a whole bunch more of what went into the original unix came from different pre-existing systems. Multics: hierarchal filesystem, swap space. RSX/VMS:the User/Group/Other file permissions mechanism. (Actually, this mechanism appeared almost unchanged in much earlier DEC OSs.)
Even the much quoted unix pipes mechanism closely resembles the 'communications files' mechanism from DTSS.
Almost nothing that made up the original unix was either completely original, or revolutionary. Indeed, the historical success and usability of *nix comes more from what they left out from Multics and the other contemporary OSs, as from what they put in. Much like Perl, their great achievement was in cherry-picking the very best ideas from many systems and putting them together as a coherent single entity that worked for programmers.
Perhaps the single biggest innovation that can be attributed to *nix, is the "everything is a file" architecture. That is what makes the unix shells work, but even that is a double-edged sword.
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.