I think it's reasonable to expect prices to increase over the next few years. They already signalled that in the Business Times article some months back (dated 6 Oct 2021) that telcos are not profiting from provision of mobile services.
https://www.businesstimes.com.sg/co...prise-as-their-tussle-for-singapore-consumers
First, the GST will rise over the next 2 years so it's already expected increases. It's much more than the 1% for us because the telcos also have to pay additional 1% for everything else, and that is passed on to us consumers to bear.
Also, the main traffic light telcos are spending money on capex because of 5G. Eventually, some costs have to be passed on to the MVNOs leasing their networks. As rightly pointed out, the main MNOs have to answer to shareholders, and shareholders will of course question why they don't raise costs on MVNO partners too, since any profits = more money for shareholders. If you put yourself in the shoes of those who stand to gain and look from their POV, it's quite a natural conclusion.
Also, I expect some form of deprioritisation of 4G traffic once 5G becomes mainstream. They did exactly that with 2G and 3G respectively. I know a few of those who stuck on 3G plans refusing to give up their 12GB entitlements and they complained about slowing 3G speeds over time - I had long moved to 4G because of the iPhone and how convenient it was to speedily access data on the go. In fact, I won't be surprised that they start to replace 4G towers with 5G SA towers to improve 5G connectivity for customers who are more profitable to them - they have no incentives to maintain a slower network for low-paying customers. It's all to push 4G customers to higher paying 5G plans.