Your reasoning states that people of a certain rating are auto-promoted regardless of the competition around them. My reasoning includes the other competition. If your logic goes through, there should also be a bracket for people with 0 LP and those who lose 2/3 of their games in the Demotion Series, where they only fight other 0 LP people, are auto-demoted. Everything needs balance, so it's only fair.
Back on logical subjects, Promotion Series is only obtained after reaching 100 LP, so technically speaking, it's not win 2 out of 3 games but rather 3 out of 4 games (the first game you had to win to get the 100 LP). That's the reason I dislike the current Promotion Series. With regard to the 3/5, I don't have much more to say; the same reasoning applies. I may have my terminology wrong, but I haven't played much during S3 anyway so I'm still learning the items.