Are there any plans for having the ArcGIS Web AppBuilder available on github? The community is building a lot of widgets and it would be nice if we could contribute to the platform (e.g. forking and pull requests). This would also make it easy for Esri to push good widgets done by the community to the core platform. It's a win win for everyone so hope you do it. There is no reason for not doing it since the entire source code is available for download.
I would say it is not that likely. The Builder portion of WAB is minified and not an open source like the stemapp portion. There are github repositories for WAB Widget out there but as many developer has different coding styles and skills, and there are legal issue with the idea of pulling custom widgets into the core WAB product.
I have requirements which require modifying small bits of the Esri controller themes e.g. having the HeaderController OnScreenWidgetIcon.js notifying when the icon was clicked. I have this requirement because I have to build a Command (not a widget with a panel or a pane) which triggers actions. Currently the OnScreenWidgetIcon.js does not have events which I can listen to so the only way I have is to create a new theme and modify the code. This can become difficult to sync in time with the main branch as I am sure the builder will evolve substantially. Having the source code in GitHub (or some of it) would make all this process so much easier but I also understand your points.
Note that you don't have to accept all the pull requests but giving the opportunity for the community to collaborate in the platform could be extremely beneficial.
I see what you are saying. One simple way to handle your need for a OnScreenWidgetIcon event is to just add 3 lines od code to it instead of creating a new theme. If you add the require for dojo/topic and then do topic.publish("eventName", someVar); in the onClick function then you now have an event to subscribe to. I have several such mods to the jimu core components and I just comment them so that I can easily find them when it is time to add them to the next version.