Especially since almost all of backwards compatibility problems are the fault of coder not the language programmer. A coders refusal to learn how to use the language in a manner that does not call for great changes in structure due to lack of polymorphism and planning is almost always the problem. Then the users of that code suffer and blame the language programmers for making changes that expose the bad coding of the software.
I'm sorry, but to expect that the Software is going to make major improvements and remain completely backwards compatible is just plain silly. If you're going to implement major changes then API changes can be unavoidable. With a language, like PHP, the whole point is to improve the language. To think you can do that without making any semantic changes between versions is inane.