If don't mind answering, I'd love it if you could provide an example or two of features that are implementable enough with support libraries, but would be a pain to build without them.
RecyclerViews are only implemented in the support libraries, even for newer versions of Android. I haven't used Exoplayer personally but by reputation it does all of the hard media stuff for you.
Most of the graphical widgets in Android apps come from the support lib.
The most complex and important is probably RecyclerView : it allows to implement recycling lists of items and replace the framework implementation.
It is not even part of the base framework since it is way more convenient to update it independently from the OS (and as a dev, you only have to handle whatever version you ship your app with).
There are shims for many of the framework features too, for exemple notifications or media.
Overall it makes the dev experience very smooth.
IMO the problem with the lack of update is mostly security
For a recent example, Google announced at I/O that it had Android architectural components in beta. They were officially release last week, and can be accessed via AppCompatActivity, and to older Android versions via the support library. For example, the method call getLifecycle(), which is associated with these Android architectural components, is new.
