In the 1970s the DEC computer company was well known for it's PDP11 and VAX-11 computers. These machines were perceived as milestones of modern computing. Sometimes, it was argued, that the reason for the success was located within the hardware, but the more important point was the UNIX like operating system. Basically spoken, DEC minicomputers were the first available system on the market which was able to run a full blown UNIX operating system. In the early 1980s, a typical DEC minicomputer had a memory of 2 MB of RAM, that was fast enough for run many terminals at the same time.
The reason why UNIX was important for customers of DEC was because they were able to write software with it. To understand the advantage we have to take a look at the world outside the UNIX ecosystem. On the high end market, IBM had produced mainframe computers which are operating in batch mode. They were running proprietary IBM related operating systems and can't be used interactively. On the other side of the market in the low end sector, Apple sold the Apple II computer which was running without an operating system and doesn't support network capabilities. The most interesting point is, that UNIX was not replaced by something which was better, but the same technology was used 10 years later. The original DEC VAX computer was replaced by modern workstations. They costs less and provided the same processing power on a small space, but the software on these workstation was Unix also. It was the same programming environment which was known before. It consists of a Unix terminal, the C programming language, tools like troff and awk and the network card for connecting to the internet.
If we are going again 10 years into the future we are in the year 2000. The former workstations which costs around 30000 US$ were replaced by cheap Intel PC but on the faster hardware a Linux operating system was installed which provided the exact feature set like on Unix workstations and on the DEC VAX computer. Independent from the hardware, and from the year, in all decades, a Unix like operating system was the most powerful software available and a here to stay for computer programmers.
What exactly is Unix? The question is asked from the wrong direction, the better question is what is the world doing outside the Unix universe. Let us take a look on the system software from other computer companies. IBM has developed his own operating systems for booting mainframe cmputers, Microsoft has developed many operating systems like MS-DOS, Windows NT, Windows XP and Windows 10. Apple had developed the old System 9 and the newer Mac OS 10, and Sega had also firmware programmed for their gaming console. All these programs were developed as an embedded system which means, that only a subset of an operating system was realized.
Let us take the latest Windows 10 version as an example. Windows 10 provides some features which makes the software useful, for example the TCP/IP stack, the commandline or the ability to install addtional software. The problem is, that many other features which are common in a Unix system are not available in Windows, for example a c compiler, a version control system, the sourcecode of the kernel and the ability to run on a mainframe computer.
From a technical point of view, Apple, Microsoft and IBM could switch to Linux/Unix. The sourcecode is available they can modify the operating system and sell a version together with their own product. We can only speculate why such an attempt is not realized. Apple for example, has explained, that Mac OS X is built on top of a BSD kernel, but Mac OS X is different from a Unix system. And it is not possible to run it on different hardware, only Apple devices are supported. It seems, that the pressure to the computer industry is not big enough and the business from the past is working great. Which means, that Microsoft and other are able to sell their own software to the user. Does it make sense, that Microsoft customers are paying 300 US$ for the latest Visual Studio IDE, if they can get a better environment for free in the Linux operating system? No it makes no sense at all, but it seems, that the customer is tolerating market failure, because he has no choice. The problem with the computer market is, that the average customer knows nothing about computers while in contrast, ocmpanies like Intel, Microsoft and Apple are the centre of wisdom and have all the information how to build hardware and software. This contrast results into a situation, in which the companies can control the market. That means, they can dictate what the customer will buy.
Sometimes it was argued, that Linux is offered without any costs, and that Microsoft endusers can switch to Ubuntu. No they can't because before they can migrate to a Unix operating system they have to acquire lots of computing knowledge. Only students of computing science have become familiar with computers. They are able to install a different operating system on their PC. For the average user the price is too high to install Linux.