I appreciate that Giffgaff rules make it very clear what will happen if you don't make or receive a call or text for 6 months - your SIM dies and you lose any remaining credit.
If you do not make or receive a call or SMS at least once in any 6 month period your SIM Card will be disconnected and you will lose any remaining credits balance on your Account. If you wish to be re-connected after this time you will be given a new mobile phone number.
With that in mind, Giffgaff is unsuitable for the emergency 'put it in the glovebox and forget it until/unless it's needed' phone* (because a Giffgaff phone would not work - in fact it'd be dead and its number gone - if 6 months had passed since it was last used)
* I have an old basic Nokia with a PAYG SIM in my car and so does the GL.
I think it's time to re-think that draconian procedure - not least because CW appear to be making a clever attempt to grab that sort of 'occasional user' market with their new £1/month contract (25 any UK network minutes and 50 texts included). Hence, for £12 per annum, a user can have an operational emergency phone and associated peace of mind.
I do not know why it is necessary to kill an in-credit Giffgaff SIM and grab back the number just because no calls/texts have been made/received but, assuming that some reason exists, my suggestion is for Giffgaff to match CW's clever new contract by allowing £1/month account top-ups by Direct Debit to keep a SIM alive.
Yes, an unused phone would gain £12 credit in the course of a year but it can be assumed that would be of no concern to the owner and of enough benefit to Giffgaff to allow the SIM an extended life-span.