Again, that’s the point.
The point of what? The entire Disney trilogy?
Yes.
Also, why shouldn’t Rey be a Skywalker (whether she is Luke’s or Leia’s kid) if this saga is centered around the Skywalker family?
Pretty simple really. In the films, the Skywalkers are the central heroes. The ST is about how the legacy of the Skywalkers and their heroic example will live on beyond their deaths. They drive this point home by having a non-Skywalker assume the name and the duties. The message being ‘anyone can be a Skywalker/hero.’