Mobile broadband

I know that I'm back in the dark ages in some respects.    In particular, when travelling, I rely on my netbook for internet access using my USB modem,  rather than an iPhone or similar.  And my frugality goes even further:  I only buy a quota of pre-paid broadband access when I need it.

For a while this was working fine.  I was using the Vodafone network, which was perfectly adequate for the places where I want to have internet access.  However, Vodafone recently changed the rules.  

Originally, your SIM remained valid so long as you topped up once every 12 months.    The new rule is that you have to top up at least once every 3 months.   As more than 3 months have passed since I last topped up, when I went to top-up yesterday, I had to buy a new SIM and establish a fresh account.  

I really wonder about the economics of this.   The starter pack is currently priced at only $10 more than the minimum top-up price, yet it involves establishing a new account etc.   Moreover, you have to go to a retailer to get a starter pack, so the return to Vodafone after the retailer's margin must be less than the return where they sell a top-up over the internet. There are plenty of mobile phone numbers available (see http://no-cobwebs-gxh.blogspot.com.au/2012/09/telephone-numbers.html), so availability of numbers wouldn't seem to be the reason.  Perhaps, however, there's a cost to carriers for having numbers linked to their network, even though they're not being used?   Or are they thinking that this policy will result in people buying more top-ups to keep their account active?    At least in my case, this is misguided, as I am seriously thinking of looking at other alternatives (Edit - there seem to be some attractive offers, such as Amaysim, but first I would have to pay to unlock my modem).