Out of curiousity: with these full rewrites, do they at least learn from the mistakes of the previous code? I can imagine there is potentially great value in that.
Kix is a drastically different architecture than Writely, so many lessons would not have been applicable. Writely was built on a contentEditable DIV, so the data model was literally whatever junky HTML the browser's contentEditable implementation would create, and synchronization was implemented by performing three-way diffs on the HTML code. It was very much a dancing bear, in the sense of "the amazing thing is not how well it dances, but that it dances at all". ...but I digress. My point is that the mechanisms and challenges of that approach are entirely different than the Kix approach. I think the important lessons would have been product-level rather than code-level.