Are you also in the employment of a network?
Maybe try to understand the facts involved here then you won't need to play the man.
The main point to understand is the difference between transient (data or time) or static (petrol) products.
When you buy a tank of petrol, like your example, your car is filled up and you take it home with you. You can use it whenever you want. The petrol you've not used yet is stored for future use.
Networks don't work like this. A network works in bits per
SECOND. Note the time component which implies the data is non-static. You cannot 'store' it.
Here's a question for you: "Why don't you take your GB of data from the network at the beginning of the month and keep it at home to use whenever you want, just like your tank of petrol?" See the problem?
If you buy a GB of data from an ISP, they have to plan that you'll use it within a specific timeframe and this is where the typical 30-days come in (up to 60-days in Vodacom's case).
To avoid all this confusion, what a network really should sell you is a bit-rate and not a fixed quantity of data. Instead of 1GB of data per month, for the same price you really should get 3Kb/s for the month, uncapped. That'll give you the 1GB (you can work it out) and this way there's no concept of roll-over.
Question is; Will you be happy with a bit-rate of 3Kb/s for the equivalent price of a 1GB bundle? I suspect not, right?
Consumers want the maximum speed, if and when, they decide to use their 1GB of data. Now the question is what's an acceptable speed? Let's assume at least 1Mb/s, though many here will want at least 5Mb/s in todays world.
So, you've bought a 1GB bundle and you want to use it at 1Mb/s. The network must now be provisioned to deliver that speed to you. That implies the network must to be configured to deliver the equavalent of 324GB per month to you and must 'buy' that capacity from its suppliers. Yet you only pay for 1GB of that.
(The above explanation also highlights the issues with uncapped services, BTW.)
But with unknown usage patterns it gets even worse. Every month you don't use your 1GB (but just might), the network should in theory up its capacity just in case you do. So the network must increase its spend because you might just use the data at some point. As you can see, this becomes a real issue if you have millions of customers on your network and is te basis for the potential price increase if roll-over had to be extended.
I hope you can by now see the problem in selling a 'static' product (a data bundle) but using infrastructure that does not deliver bundles to supply it? And the best way to bring these two concepts together is to put an 'expiry' on the bundle, i.e. make it semi-transient.