When Linux appeared in the first half of the 1990’s, I used to hear a lot: „yes, this is a nice thing, but it is not a real Unix“.
So why was there a different name, even though it was behaving almost the same? It was Posix, but Unix was a trademark, that could not be applied to Linux.
Unix was very important in the 90’s and in the 2000’s, but now it has lost almost all of its relevance.
Newer systems almost always use Linux and systems that still run Unix (Solaris or Aix) are usually considered as something that needs to be migrated to Linux sooner or later.
There is nothing wrong with Unix. It brought us great concepts and these concepts are relevant today. And inventing and standardizing these concepts was a good thing and a success story. But all the good stuff can now be found in Linux and since the progress is happening there, it has surpassed the Unixes. Of course it was a factor that HP in the late 90’s or so announced that they saw no future for their HP/UX, which they revoked, but it had created damage to that system. Oracle had bought SUN and that weakened Solaris. Many other Unix-variants have already lost their relevance long ago or are still lively and good niche systems, like BSD.
The success story behind this is that standards that work across companies have been established and allow systems to work together and to behave similarly.