If your webapp is simple a rewrite is no big deal and often cheaper than updating the old. As your project gets large that is no longer true. I work with embedded systems, when everything was small ( 8 bits isn't enough for anything else - new feature often means removing something else) we of rewrote large parts to get one new feature. it was easy to estimate a new project and we came in on time. As projects get bigger (32 and 64 bits are now available) we can't do that we can't afford a billion dollar project to rewrite every year.