In reply to Phillip Hawley.

Plugins will fall into disuse if their developers stop working on it. This has absolutely zero to do with how well WP scores on backwards compatibility. Third party code just needs to be maintained and the truth is with tens of thousands of plugins this isn't always going to be the case. The majority of update issues are dealing with the reality that 3rd party developers are not maintaining their code in time with major WP updates, not because it has been made impossible or hard. For end-users the answer is to financially support the plugins they use and to test major updates first, especially if their site is a business site.

If changes are made that are hard to overcome or require entire rewrites then perhaps you could rmake a case that backwards compatibility has become an issue. Have you got some concrete examples of that? I can't really think of one earth shattering update that has been a net-negative. Keep in mind most 3rd party developers want the API to get more powerful, quicker.