Almost no such thing as a lock in an RSA side. Almost all the best options are unproven in one way or another. Either role the dice a bit (era, opponents, number of tests, team balance, playing role...) or accept someone worse. Even if you pick the best (Pollock was a better player than Donald, and possibly than Steyn in my mind) it doesnt work out well.
Pollock is the 4th best RSA RA quick at best in my book, with Kallis already playing. He is the 3rd best RA quick AR, which doesnt put him in good standing. Too much competition, especially when factoring in the other types of bowling options, particularly when there are AR options.
If I didnt fancy Procter, I would likely find a spot for him because Im a huge fan of ARs, but Im not sure I would call him a lock even without Procter.