To me it seemed Sky+ became 'free' with any package when there was significant numbers of comparable PVRs available on the market - it became difficult to justify the 'value' of £120 a year when you could get a similar service for free.
Sky+ is a service which doesn't give you any extra channels - you are only entitled to pause, record, rewind, and series link the free channels and channels that you already subscribe to.
The HD Pack gives you fifteen channels which are not available anywhere else - maybe a handful are available via Virgin - plus, if you subscribe to Sky Sports and/or Sky Movies, up to another dozen high definition channels which aren't available anywhere else.
Sky don't give the Variety Mix away for free. Sky don't give the News Mix away for free. Sky don't give Sky Sports away for free. So why would they give one of their other packs away for free? Surely giving the HD Pack away for free would just persuade people to stop subscribing to the Variety Mix ("Hey, if I get Sky One HD for free anyway, why bother subscribing to the mix which gets me Sky One SD?"). Which would mean that Sky would not only lose money by not charging for the HD Pack, but might also encourage people to drop their subscriptions to the SD Mixes as well. Double whammy! And not in Sky's shareholders minds in terms of making profit.
The only thing that Sky+ and the HD Pack ever had in common was the initial cost - £10 per month for each. That's it. Nothing else about the two is the same. One is subscription to a set of channels - one allows you to pause and record what you were already paying to watch.
They are two totally different services, and if the HD Pack had originally cost anything other than a tenner a month people wouldn't be trying to claim that because one eventually became free the other one must as well. If the HD Pack had cost (say) £16 per month, putting it on a par with Sky's two premium mixes, people wouldn't be claiming that it should soon become free - because nobody is claiming that Sky Movies or Sky Sports should soon become free.
Anyone who thinks they'll drop the charge fairly soon (or indeed ever) really needs to stop comparing it with Sky+ and start comparing it with every other subscription package that Sky sell on a monthly basis - none of which are ever likely to be free. And when Sky are pretty much giving away the HD boxes at the moment, on the understanding that anybody who gets a free or massively discounted HD box must subscribe to the HD Pack for at least twelve months, they are even more unlikely to suddenly stop charging for the HD Pack.