> That the pyramids were plundered by tomb robbers is perfectly
> plausible, but it is not proven in most cases.
> Consequently, I consider it equally plausible that some, if not
> all, of the pyramids were cenotaphs and not actual tombs.
> As to where a pyramid-as-a-cenotaph’s king or pharaoh was
> actually interred is another matter.
The south-tombs in the funerary complexes of Djoser and Sekhemkhet could be considerer as "cenotaphs". They consist of a mastaba superstructure and a miniature version of the subterran corridors and chambers under the pyramids. No sarcophagi though.
In the 4th dyn, the south tomb has disapeared from the funerary complex, but "cult-pyramids" appear. Here we have a miniature version of the main pyramid but the internal / subterran layout doesn't immitate that of the main pyramid. No sarcophagi in the cult pyramids.
In the Middle Kingdom we have the Bab el-Hossan (in front of the temple-tomb of Mentohotep II at Deir-el Bahari), here we find a symbolic burial of King Mentuhotep II in the form of his famous statue now in the Cairo Museum, this statue was even wrapped in linnen like a mummy.
From the middle kingdom on we have so-called "cenotaphs" of officials at Abydos, but these have no subterran part and thus no sarcaphagi.
The practice of cenotaphs from this point on is closely connected with the cult of Osiris who was especially venerated at Abydos. The Osiris-tombs themselves are a form of cenotaph, these do include an empty sarcophagus. The Osiris-tombs that have been found date from later periods, most famous are the Osireion at Abydos and the Osiris-tomb under Khafre's causeway at Giza.
Ahmose (last royal pyramid builder) also had a cenotaph at Abydos, the subterran part (no sarcophagus) however was not under the pyramid but a distance away, the complex also included a pyramid for Tetisheri his grandmother (room in pyramid with stela, no sarcophagus).
Closeby is the Middle Kingdom funerary complex of Sesostris III, by some also considered a cenotaph but by most archaeologist considered to be his real tomb. The Abydos subterran complex with sarcophagi has many means to misguide / decourage tomb robbers while the subterran corridors under his pyramid in Dashur lack the usual blocking systems.
Appart from the pyramid of Sesostris III at Dashur there's no indication that any main pyramid containing a sarcophagus wasn't a tomb. A cenotaph doesn't need security measures against tomb robbers since there's no body and no grave goods (hence the lack of a blocking system at Dashur).