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.