Mac OS continued to evolve up to version 9.2.2, including retrofits such as the addition of a nanokernel and support for Multiprocessing Services 2.0 in Mac OS 8.6, though its dated architecture made replacement necessary. Initially developed in the Pascal programming language, it was substantially rewritten in C++ for System 7. From its beginnings on an 8 MHz machine with 128 KB of RAM, it had grown to support Apple's latest 1 GHz G4-equipped Macs. Since its architecture was laid down, features that were already common on Apple's competition, like preemptive multitasking and protected memory, had become feasible on the kind of hardware Apple manufactured. As such, Apple introduced Mac OS X, a fully overhauled Unix-based successor to Mac OS 9. OS X uses Darwin, XNU, and Mach as foundations, and is based on NeXTSTEP. It was released to the public in September 2000, as the Mac OS X Public Beta, featuring a revamped user interface called "Aqua". At US$29.99, it allowed adventurous Mac users to sample Apple's new operating system and provide feedback for the actual release. The initial version of Mac OS X, 10.0 "Cheetah", was released on March 24, 2001. Older Mac OS applications could still run under early Mac OS X versions, using an environment called "Classic". Subsequent releases of Mac OS X included 10.1 "Puma" (2001), 10.2 "Jaguar" (2002), 10.3 "Panther" (2003) and 10.4 "Tiger" (2005).

What did dated architecture on the Mac OS line make necessary?