I fly out of SEA, and as a 75k I get to select PC during booking, but what I have noticed is that there are always several of those seats already taken (thankfully mostly window seats). Since I am typically up at the top of the UG list I am not sure if there are that many 75ks on my typical flights out of SEA or if people are purchasing the PC seats outright at time of booking. I do book typically 5-10 days out, which probably makes the difference.

Doesn't surprise me that 5-10 days out there would be PC seats taken for SEA-departing flights, either by other 75ks who have unrestricted access or by MVPGs buying expensive enough fare classes. Or even regular folks who value space (aka me in a couple of years).