We generally don't do that because we can't justify it in terms of the business
True, but Joel's point is that a rewrite can actually be bad for the code not just the business, because you throw out years of real-world testing and bugfixes.
And to be honest, unless Textmate 2 is significantly worse than 1.x, I'll buy an upgrade license.
Very true, but you also get to trade in on the experience and information gleaned from the use and development of a product over many years. I think it is safe to say that Allan knows far more about Textmate now than he did when he started it, and perhaps that insight has allowed him to bring some significant improvements to the text editor as a whole.
Maybe I'm just biased; I tend to rewrite stuff before I've even written it once.
I think that applies on project with multiple developers and whose developers are not in the company anymore. When it's a solo developer, he still has the experience and doesn't lose so much from throwing out the code
I've learned this the hard way enough times to make it stick to the point now that even when I'm just writing regular, mundane CSS, I make sure to comment in every workaround I write, no matter how routine. It takes longer, but it's worth saving me from myself.
Code very much does get stale, at least to the developer looking at it every single day, and sooner or later almost everybody wants to throw everything out and start over. We generally don't do that because we can't justify it in terms of the business, but when you're a solo developer I suppose you can do whatever the hell you want ;)