I do not know off the top of my head how many employees work exclusively on SketchUp and I’m certainly not authorized to publish any company information (I am just a developer) but I think I am safe to say the following.
I sometimes see this idea that SketchUp development is slow because Trimble leadership has made a conscious decision not to invest in it. From my perspective that simply is not true. As @ateliernab says, the coordination of a large number of people is challenging, as is the nature of maintaining a large application with a long history, a broad user base, a powerful extension API, and multiple platforms.
Could we do better? Sure. Any software development team that thinks it cannot improve is in trouble. However, this conspiracy theory that Trimble does not care to invest in SketchUp for the long term is just silly.