the reason to rewrite depends on the project your involved in and what features you want to make use of moving forwad. A simple example when .net 2.0 came out,

the reason to rewrite depends on the project your involved in and what features you want to make use of moving forwad. A simple example when .net 2.0 came out, simply upgrading to the latest version of the framework didn't give your older apps all the benefits of latest features included in 2.0 (new controls, master pages, provider model support out of the box etc) ; A rewrite was inevitable. When MVC was introduced everybody in the .net community felt it was their sacred mission to promote it over webforms. If you wanted to stay current skillwise and evovle with the community you either rewrote if this was even feasible or used MVC for the next project ( for most this really meant a new investment in learning and also not being able to reuse a vast array of controls and component that were already written for webforms and available, instead we were reinvesting our efforts in rewriting in such a way that this was consumable from an MVC paradigm). I agree with you that a lot of this is just hot air, but not if you are seriously invested as in someone writing tooling for webforms for instance and your business depended on it, your customer base dimishes quickly and you have to quickly adapt and rewrite. This is the same for winforms/wpf/silverlight. People seriously invest in these technologies after going through a lot convincing top management and what not because MS promised at some point that they are committed to seeing this through. Consider having a product developed in silverlight. Now MS is saying they are backing html5 but fear not, you can still rewrite it for metro because sadly IE10 on metro will not support plugins. This is their own plugin on their own operating system. Not supported! Which is not the same thing because SL atleast worked on the mac cross platform. Your metro app wont. So, html5? even still your developing a metro app so forget cross platform or perhaps you can reuse a lot of the code base but some effort is still required and we're really hoping for the best. In the end we are rewriting again.