Deadlines are not arbitrary, they’re a promise we make to ourselves and our users that helps us rein in the endless possibilities of things that could be a part of every release. We aspire to release 3 major versions a year because through trial and error we’ve found that to be a good balance between getting cool stuff in each release and but not too much that we end up breaking more than we add.
Good deadlines almost always make you trim something from a release. This is not a bad thing, it’s what they’re supposed to do.
The route of delaying a release for that one-more-feature is a rabbit hole. We did that for over a year once, and it wasn’t pleasant for anybody.