Well, / is path traversal; I think + would be too easily conflated with the “similar but importantly different” string concatenation, especially since « + "/" + » is an idiom in string-based path manipulation. For instance:

  p1 = "/a/b/c"
  x = "d"
  p1 + "/" + x == "/a/b/c/d"
  p2 = Path("/a/b/c")
  # If + replaced / for Path:
  p2 + "/" + x == Path("/d")

Now if you've just reflexively written the concatenation the way you did before, the type signatures match up but the result is completely wrong. The current situation where / doesn't exist for strings and + doesn't exist for Paths is much safer versus that.