I could buy this if not for the fact that back when UNIX was created, there were already better operating systems and sane solutions to those issues existed. It's more like that those aspects simply weren't really thought through, but instead just hacked together.
Contrary to what seems to be a popular opinion nowadays, UNIX wasn't the first real operating system, just like C wasn't the first high-level programming language. I knew I actually believed the latter, due to the way C/C++ many books were written. But no, in both the worlds of programming and operating systems, there already were better thought-out solutions. It's a quirk of history that UNIX and C ended up winning.
There were differently thought-out solutions, but not necessarily better or worse. Perhaps the issues they thought out didn't matter as much at the time and very unlikely to matter even a little bit today, and who knows how much they got wrong and much worse. It's very hard to speculate. But one thing I'm sure of is that these things cannot be really well designed from scratch and all the problems manifest only once they are used by people. So widely used systems can only be compared to widely used systems, not something niche or unused.