Modern operating systems like windows and linux were created decades ago, and throughout their history, they accumulated a lot of legacy and issues related to them. The main problem is backwards compatibilty. It is impossible to make major changes to the system components which could actually improve the system, because doing so would break countless of software which rely on that specific behaviour. Also, while the design architecture of those systems was very clever decades ago, it might be unefficient by modern technologies and system design philosifies. Of course changing the system architecture at this point is obviously not possible.
So what if a new fully-featured operating system was created today from scratch, with modern software and hardware technologies in mind. How could it compare to the existing systems in terms of design architecture efficiency, performance, security, user and developer experience, etc? What would be the main differentiating features of this hypotetical system? What would be the main drawbacks of it compared to old systems, not counting lack of backwards compatibility with old hardware and software, and lack of widespread adoption amongst users and developers?
It’s still partially true. While consumer stuff is constantly being updated and rewritten, a lot of corporate stuff is stuck to whatever system it was originally designed for. Mainframes are still around for a reason