Indeed. My intuition based nothing other than my subjective experience is that there are times that throwing away the code is the correct decision, but they are a rounding error compared to the times that people want to throw away the code, so to a first order of approximation "never start over from scratch" is correct.

Simply shifting the burden of proof to the "rewrite" side is usually sufficient. Where I currently work a rewrite request is not answered with "no" but "show me the data." 90% of the time the requester doesn't even try to gather data, the other 10% of the time results in some useful metrics about a component that everyone agrees needs some TLC, whatever the final decision.

> we can say that Gecko ended up technically better than incremental changes to Internet Explorer

> ...

> That's hardly conclusive but it's some evidence.

Given Internet Explorer's monopoly position, and consequent disincentive to compete, it's not really the best comparison.

Compare to something like Opera Presto, a codebase that - while younger than Netscape - predates Internet Explorer, which underwent incremental changes while remaining in a minority market position. It was killed by market forces, but I doubt anyone would contest it was a badly put together piece of software in the end.

Konqueror is another example. It's not quite as compelling, as KHTML itself has fared less well than its forks, Safari WebKit has never exactly been a leader in engine advancement, and Chrome's innovations, while incremental, were largely built on a scratch-rewrite-and-launch-all-at-once of one component (V8). However KHTML/Webkit/Blink is still pretty much an incremental rewrite success story.

Yeah but Microsoft let IE stagnate for half a decade after it had achieved dominance. There were 5 years between IE 6 and IE 7. If MS hadn't disbanded the IE team but had kept up the pace of incremental development, I very much doubt Mozilla would have ever been able to catch up again.

I would say Joels point still holds in the general case where you you can't count on competitors to just stop developing their product until you have achieved parity again.

And, as far as I know, Microsoft has developed Edge from scratch after all. The years of incremental updates to IE are now maintained for legacy support.