Companies utilizing public domain characters isn't anything new or out of the ordinary. Its why you see new Dracula and Frankenstein movies every year. This recent advancement on public domain means a lot of classic pulp characters and mystery stories are entering the public domain, so there's good reason to be excited.
That said, Zack is delusional if he thinks his public domain short story anthology will be anything but a disaster. Maybe if he pulled an Antarctic and made some transformative media like a new comic series combining old and new characters he'd have something. But writing single-digit page fan fiction about them is laughable. (just read the originals--they're still good!)
Antarctic's not-Mary Marvel, Mary Miracle. Please don't sue. Original Character, Do Not Steal.