[ikurei]

Webapps are rewritten because a developer wanted to use the new shiny, or someone was convinced that everything will be better with the newer frameworks everyone is using. Also, it often goes hand in hand with giving it a more modern look-and-feel.

But the point is not whether webapps are rewritten, but whether they have to be rewritten. I know some old enterprise webapps made with PHP about 10 years ago that are still working fine.

You do have to worry about security issues, and the occasional deprecation of an API, but there is no reason why a web-based service should need to be rewritten just to keep working. Is that true for mobile and desktop apps?

[bluGill]

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.

[pixl97]

>but there is no reason why a web-based service should need to be rewritten just to keep working

I mean most webapps of any size are built on underlying libraries, and sometimes those libraries disappear requiring a significant amount of effort to port to a new library.