I'm running out of ways to say this, but just to emphasise, hacking internal APIs will end in disaster. It already has for us in the past, and this point is obvious to experienced software engineers. It is the whole reason encapsulation was invented. I think it is likely that in 2030 and beyond we will still be dealing with painful compatibility issues caused by the internal engine hacking that people are doing now.
Providing source code is not the same thing as providing a stable API. Other tools which have a public, documented API, which they change over time, sometimes in backwards-incompatible way, is not the same as letting people loose on the internal engine. That example of public APIs that evolve over time in a responsible fashion is in fact the thing we want to aim for. As I said, that's the industry standard, and what Construct has at the moment is a uniquely bad situation due to the poor encapsulation of JavaScript. This is not just a technical point: it is a matter of support and maintenance and what you promise customers and developers. I don't think everyone here has fully appreciated that.
If everyone is insisting we do something that has and will end in disaster causing both us and our customers to suffer as a result, and insisting that actually don't worry, everything will work out just fine - sorry, software development for a tool used by hundreds of thousands of people that is maintained over a span of more than a decade just doesn't work like that. Our own experience has shown that, and it's widely understood stuff in the software industry; if you don't accept that, I don't really have anything more to add, and so I will stop responding to this thread accordingly. I am so convinced by this point, that I would heartily endorse all our competitors doing what you are asking us to do, because it will end in disaster for them, and if we can avoid that then it will turn in to a competitive advantage for us.
If we put the addon SDK behind the scripting interface, then we solve our problem of how to have a public API with encapsulation that we promise to support in the long term. This can be thoroughly and widely used and will most likely not cause any disasters even years down the line. I know this does not give unfettered access to the engine, but as I say, that's the industry standard when it comes to API design (not source code), and it still gives you a lot - there is a broad and fully documented API there. Then we have a single place to add new APIs which are instantly accessible to both scripting users and addon developers, which is much more manageable than having to effectively maintain and document two separate APIs, and means we can move quicker and expose more APIs in the long term. I think that is the best direction to go in, and I think we will start to look at a "v2" SDK along those lines in the near future - as the sooner we can move away from the serious risk of disaster that we are currently facing, the better.