In the case of UNIX, it's not merely commercial success and popularity. It is the choice of a large portion of the smartest people working in technology. If we were talking about success amongst the hoi palloi (like, say, Windows), then your point would stand...but we're not. UNIX has seen its fortunes flounder and flourish multiple times over the span of 30+ years, and yet, today, UNIX runs the most important technologies on earth (Google, Amazon, the majority of all other web applications, the best smart phones, most smart devices with anything more powerful than a simple microcontroller, and a whole lot more), because the people building them chose UNIX.
So, is UNIX technically sound and superior? In the general case, I would say, unequivocally, yes.
Kernighan and Mashey mentioned in "The UNIX Programming Environment" (1981): "Success or failure often depends on nontechnical factors, whose importance often goes unrecognized by those who evaluate systems on purely technical terms." And they go on to mention some of these nontechnical factors, such as having started out on the DEC PDP-11.
In Ken Thompson's Turing Award speech, he claimed: "I can't help but feel that I am receiving this honor for timing and serendipity as much as technical merit. UNIX swept into popularity with an industry-wide change from central main frames to autonomous minis. I suspect that Daniel Bobrow (1) would be here instead of me if he could not afford a PDP-10 and and had to "settle" for a PDP-11."
http://cm.bell-labs.com/who/ken/trust.html
Similar fate apparently happened due to the specialization of the MIT AI Lab's ITS: "Of course, ITS wasn't portable by any stretch of the imagination. It was an operating system in the old school of development, and it died rather suddenly when DEC announced that they were discontinuing the PDP-10 and its descendants in favor of the PDP-11 and VAX systems."
http://www.crackmonkey.org/unix.html
(Personally, I only use a UNIX because it's a tool of the trade. It's free, or built into Macs. I don't enjoy it, but I use it, and found the _Unix Hater's Handbook_ refreshing.)
http://www.cs.washington.edu/homes/weise/uhh-download.html
In particular, the "X-Windows Disaster" chapter of the Unix Hater's Handbook should be required reading for anyone considering GUI work on a UNIX-derived platform--and is also absolutely hilarious.
Eh, making light of something's flaws is not the same as denying its value or strengths. I haven't been a desktop linux user for 5+ years because I hate the X Window System.
Did you read the chapter I linked to? It really is quite entertaining.
So, is UNIX technically sound and superior? In the general case, I would say, unequivocally, yes.