At the risk of stating the obvious, just accept that this golden goose is cooked and gone. You are welcome to argue your rights and what DBS can or cannot do, but the reality is that they have a bunch of well-paid lawyers who drafted the T&Cs to protect their position and made sure they are able to do what they want without contravening MAS regulations or any code of conduct. If you think about it, it is therefore not surprising why all official comms has been crouched as
clarification & not change of T&Cs.
Honestly, given how indiscrete members of this and the flyertalk forum are, as well as the glory seeking bloggers talking incessantly about using this card with prepaid cards, it is a surprise this has lasted as long as it did.
Perhaps you might think I and other old-timers are selfish for no longer sharing on this forum, but the reality is that a lot of what we do to maximize miles earning is not in line with how the banks had envisaged their cards to be used. When it is not well known, not many people do it and it skirts under their radar. When everyone starts talking about it (or hinting about it or trying to be clever and thinly disguising what you do with obvious nicknames), many more people jump on the bandwagon and the same transactions appear on multiple people's statements, you can be sure their analytics will pick it up and the bank will close the loopholes.
Anyway, going to take a little bit of my own advice and /rant off. For my old buddies (you know who you are!), hope you are still enjoying those flights in the pointy end of the plane.