Instead we get 100+ different OSes. What good is that? Will a program I write for OS Number #37 work on OS #78? Maybe. Maybe not.
The same nonsense is going on with regard to computer languages: rather than expanding around existing proven languages we get a plethora of languages with a mixed bag of advantages and downsides that don't constitute real advancement. C++ makes sense as an expansion of C. Python is just a scripting language gone mad.
I know. I know. Python is popular and "powerful". But so is Justin Bieber and Lady Gaga.
You thought incorrectly.
Computer languages, like OSes, are created to fill specific needs. Neither one can be all things to all people. That concept is foreign to IT, and computers in general.
COBOL is geared towrds business / large amount of data in multi-user environments. Fortran is geared towards scientific computing. PHP is programming for web sites, while SQL is aimed at databases. It would be impossibly clunky, if not downright impossible, to design and implement a computer language that would be good at every task.
The same thing goes for an OS. Windows is aimed at people who don't know anyting about computers, don't want to know anything about computers, or just want a plug and play system and they don't care how the computer works. Linux is aimed at people who care about how the code runs, who want the ability to do tasks that cannot be done on Windows, who interact with datacenters, mobile devices, and desktops at the same time.
Different OSes for different needs.