I'm repeating myself here, and there are only so many times I will repeat myself before I deem it no longer helpful to respond. Here we are talking about the long-term maintainability of installable addons, not whether you can download a copy of the engine source code. Any open-source project with installable addons will also have to consider long-term maintenance of the addons that are created, and the professional, industry-standard way to do that is encapsulation, which is what the Addon SDK v2 does.
I don't think the issue is really with open source, and while I agree encapsulation is generally a good thing in a large code base. I think the frustration comes from the fact that with the changes, the API surface is being heavily reduced. compared to other similar game engines which also use encapsulation, the main difference is they all have vast API's to modify the engines internals. and while these are generally not supported indefinitely, they are supported for a subset of time usually gated by major releases. this probably does not play well with construct release cycles since they are very frequent. but access to all the older version of the editor still help.
one thing that worries me as an addon developer, is major overhauls like this will probably occur in the future again. and a large portion of the community hard-work will need to be rewritten or become obsolete.