You could chuck in $20 every time you go on holiday and get 500mb...if you didn't use the whole amount, it would stick around until the next holiday.
This. I could put in $20 and it'd last for more than 6 months. Once or twice I had to top up because the credit was going to expire otherwise
I think you have both answered the reason why the plan got grandfathered. It has been around since XT launched, and I had personally thought it had already been shutdown.
If you are only topping up $20 every 6 months or every year just to keep it alive I think we can all agree it's an unsustainable plan. No one else on the market does it (even for carry over data plans you still need to be on some sort of plan to get it).
I think the one-off data extras you can purchase via the smartphone app and top-up that way too are the way to go, that way you just purchase what you need for that period you are going to use, and set the data caps via Mobile Self Service.
I personally love the smart one bill accounts as that is what I have for 3 data sim cards for various devices I own. Set the smart cap to zero so most of the year you don't get inadvertantly charged for casual usage when you switch them on, then add a one off extra when you need to use it and it gets charged to your main account. Couldn't get any easier than that IMHO.