CUPS will not be entirely replaced. It will still be what talks to the printer, picks the right drivers, manages the installed printers, etc.
printerd is more like a session print spooler. It lets you enumerate printers, submit jobs to them, etc. If the printer was a network (ipp) printer that supports pdf, or a cloud service like Google Cloud Print then everything will run without any instance of CUPS.
However, if you're printing to a local printer, or a printer attached to a print server (on linux) then printerd will hand the pdf over to CUPS, running locally or on the server.