2 pointsCore is always evolving and getting better, it will eventually end up in a place where everything can be done without core being affected at all. To get to that place means some "hurt" for existing addons. What we have is a small team of addon developers who are actively involved in helping Matt @ecartz and myself to "bend" the core code to better suit their addon needs, which means their addons become much easier to install. Think back to the days of 2.2 (or even 2.3) where you would see addons changing core code on lots of different pages to do something. We've massively reduced that with the help of supportive addon makers and supportive shopowners. On the flip side of that coin are the addon makers and shopowners who have zero input into the core code. If one has zero input, one should expect to have to deal with problems as they arise, rather than know upcoming problems in advance. I can summarise this as so: The core does not stand still because of addons.
2 pointsThe changes in Phoenix will sometimes change the code to where it is no longer recognized by the code in addons. For example, a function may be moved to a class so the code in the addon can't find the function any longer. There's no way for developers to plan for this so we just have to react after the fact. But as soon as we upload a new version of the addon, the code in Phoenix changes and breaks the addon again. Testing the addon with the new version of Phoenix, identifying the new problems, fixing the code and then uploading the new version of an addon is not a small job so upgrading can't be done for every version of Phoenix. Unfortunately, there's just no way around it.