To be fair, promotion to CAPT or CDR is still pretty competitive, and maybe arguably LCDR depending on your community. Agreed about the O-3/O-2 realm though......I can't imagine that was every very competitive, even if there was a more formal process. I'll ask my old man, he pinned O-3 in 1962 so that would probably be a good estimation of the history of competitiveness....
He did say 20 years ago. 14-16 years ago, making O-4 was less complicated than going through a TSA line. Making O-5 required getting the right boxes checked and some career inertia. All I'm saying is that 20 years ago, I don't think it was nearly as competitive as today and not as "intense" as perhaps was conveyed.
Obviously CO/XO (for any URL) can be a different matter.