So judging from the responses, I guess the conclusion is that I can't simply estimate the availability based on different subclasses availability. I am often the victim of such harsh "inventory control", as I have never bought "Y" ticket.

As some of you suggested, I usually never give up, and check the availability on the ITN for my specific subclasses almost every half day when I need to travel. Once I see the preferred flight "open up", I pick up my phone and call MPC desk to have my flight changed immediately, because I know they don't "last". I've seen a inventory suddenly drop to "0" in all classes in matter of hours, but in 2 weeks time, they all changed back to full availability. Wonder what's the trick behind, and who's behind.

By the way, the particular ticket I'm concerned is ex-Australia in "K" class. I have an option of paying extra AUD$320 for "B" class if the other is sold out. In the past, I was frustrated with "M" class's limited avalability (ex-Canada), and now I understand why, cause it's the same as ex-Singapore's 2 week excursion fare! The good news is that if "K" in Singapore is used for 1 year open ticket, I should expect a more generous availability when I book my ticket in OZ in this subclass!