If I assume correctly this development is not changing the structure of the database itself, isn't it? Maybe, sometime in the future, when even multithreaded approach won't suffice, a quantum leap into sharded blockchain will be necessary.

That's something we've been planning for a while - the fabric architecture. This is a step toward that future.

A side question - does this breaking of the code into plugins and apis means that community based nodes will be possible serving specific plugins only.

That architecture is already in place now, and different types of users can run different sets of plugins depending on their needs. :)