I somewhat had an inkling that this would happen—the day System 10 moved to AMD64, it was going to happen, sooner rather than later.
If Apple had decided to use System 9 as the base for System 10, would we end up radically different from where we are today, in terms of features, User Experience, &c?
They could have improved the memory manager to a greater extent; since Freescale 68k-based Macs were already obsolete by 2001, there was very little need to maintain compatibility with it, for example.
They could make a break from the past—but it didn’t have to be as dramatic as it turned out to be.
In any case, the UNIX integration, and the subsequent cut-over to AMD64 were big mistakes in terms of security.
WOW! You really don't know what you are talking about, do you? UNIX is demonstrably one of the most secure operating systems in the world and putting OS X on top of it was not a mistake. Apple OS X has not had a single viable computer virus invade OS X in 17 years since OS X has been in the wild (OS X server was released in 1998 into the wild). MacOS 9 had 139 known, truly viable computer virus when it was discontinued, plus many variants, and additional Trojan horses programs. OS X has only 57 known trojan horses. That is secure and safe. The latest vulnerability is in the EFI boot loader which comes into play before UNIX even boots. . . so it cannot even truthfully said to be part of UNIX and would effect every OS regardless of security tightness.