They are trying to force people to eventually buy the pro version. Sketchup Make will eventually become obsolete and inefficient compared to the newer versions. Eventually, newer plugins will become incompatible with the old SketchUp make.
Let's not kid ourselfs. Sketchup free is NOT a replacement for make (at least in its current state). They completely misunderstood what made SketchUp so attractive in the first place; it was its ease of use and how powerful it was thanks to its plugins. Without these extensions, sketchup would have been forgotten in obscurity and it would have never achieved the acceptance it has today. Only simple models would have been possible and it would have been too inefficient for anything more complex. These extensions were easy to use and fit into SketchUp's philosophy well.
Sketchup free is a purely business move. It has been completely gimped in order to force people into pro subscriptions. They took away features under the excuse of making it easier and more accessible to use. While truthfully they made complex models even harder to create. Even an advert supported Sketchup make 2018 (advert on startup) would have been a better idea then the online free version.
The current trajectory will most likely lead to a reduction of hobby users which would eventually reduce the number of extensions in development. Which will lead to a decline in pro users. Even kids will move on from FREE as they eventually will want to create more complex creations (when I was 14, it was only a week before I downloaded my first plugin).
And even if someday they add extensions to SketchUp Free, it's online only.