I don't think they will increase much for the $10 plan. On the business perspective, they would want to increase their ARPU. Simba over the years has seen steady rise in ARPU.
I think they are luring people to upgrade their $10 plan to $12 plan with an extra 4GB International Roaming. So in this way, they will increase their ARPU and be more profitable to challenge the other 3 MNO (Singtel, M1, Starhub).
If Simba upgrade the $10 plan to 2GB international roaming, more people would stay instead of upgrading. So in this way, their ARPU won't increase much.
This tactic is smart like our fast food, upsize and no upsize difference very little. But the benefit to consumers is a lot, then businesses earn from this little increases.