Joel Spolsky's essay is more about rewrites of "ugly" code. E.g. Fix the disorganized code by gradually refactoring into cleaner modules over time instead of a total blank slate rewrite.

In contrast, this Geoff Greer essay is about paradigm-shifting software architectural changes which by their nature, are very difficult to retrofit into an old existing code base. For these, it's often easier to start coding with a "blank slate" rewrite.

As examples Geoff's category of rewrites (new architecture paradigm) vs Joel's category (unhygenic code cleanup), we can look at Joel's ex-employer Microsoft:

- SQL Server database diverged from original Sybase code and rewrote the engine. One of the architecture changes was switching from page locks to row-level locks

- C# compiler completely rewritten from C++ code base to C# codebase. One architecture change was "compiler exposed as a libary service" instead of being the closed "black box" that was assumed by the C++ code.

- operating system: MS Windows NT as a blank slate operating system instead of gradually extending old 16-bit DOS code or 16-bit Windows 1.0 code. One motivating architecture change was the new 32-bit protected mode on the newer Intel 286 & 386 chips. Another factor was the switch from "cooperative multitasking" to "preemptive multitasking".