I would not call the SketchUp team lazy. They do a ton of work to improve the life for us developers by fixing bugs, adding new API functionality and improving documentation. Also it’s not really true that extension developers do the heavy lifting. It’s quite easy to get an extension to work but it requires a ton of knowledge to design a good and future proof API. Many of the popular plugins are quite poorly made when you take a closer look at them but still do their job great. The core and API however need a much higher level of quality to assure they can be improved in the future while still supporting existing extensions.
The reason why these tools don’t feel like integrated parts of SketchUp is not because they are made by third party developers, but because these developers lack knowledge in how to make something look, behave and feel “SketchUppy”, with the exception of Skalp that tries to emulate access to the graphics pipeline by drawing physical geometry to the model. As an example Solid Tools is a native feature but still doesn’t feel very integrated as it doesn’t honor components.
Rather than integrating functionality that a lot of people have no use far (and having it designed by people that don’t use SketchUp) I think there should be made an effort to spread knowledge in what makes SketchUp feel like SketchUp. It would also be great if Skalp had access to drawing to the view directly, instead of having to draw physical geometry to the model, assign layers to it etc. With this API improvement Skalp could be made to feel just as integrated as the native section fill.