It can be argued the other way round. POSIX is tech debt and exists in modern machines so widely mostly because there happen to be free implementations of it. My experience from interviewing has been that a remarkably large number of developers actually never interact with POSIX directly these days: they never work with files and never open a socket or fork process trees. They certainly never use UNIX-adjacent stuff like X. Instead they interact with APIs layered on top like NodeJS, Java, C#, Android, Swift, HTML. None of which bears much resemblance to POSIX. Like, if you read about how to work with the network in iOS or Android you won't be directed to POSIX APIs.