With that phrase, you may have suggested the answer to the "no new phyla" problem (or perhaps it should be called an observation rather than a problem). If all species are related by common descent, we should expect to see a gradual branching from common ancestors. And we do see this. It forms the tree of life. But you are objecting (if that's the word) that there is no new trunk forming, or at least no new major branch being developed.
In the Cambrian period, living creatures were quite small. I suppose the array of what existed may be analogous to life at the bottom of the oceans today. New forms actually may pop up from time to time. The variety of tiny creatures is amazing, and no one has catalogued them all. Some may be relatively new.
In any event, it's difficult to see how a new body form, if such shows up, is going to start developing enough descendants of various types to get recognized as a whole new phylum. The precursors would probably get gobbled up before the process developed very far. The existing "tree of life" may be hogging the show. So be glad our ancestors appeared when they did.