For $20 you can own a 64gb flash drive:
http://www.amazon.com/PNY-Turbo-64GB-Flash-Drive/dp/B00FDUHDAC/This is over double the current blockchain size.
If you're doing long-term storage, my guess is you've got more than 1 BTC that you are storing. $20 shouldn't be much to ask for.
While I understand the concerns, I find it likely that $20 will continue to be sufficient for at least holding the blockchain for 5-10 years. In that time, if it gets out of hand, it'll mean adoption has increased, not decreased, and the price along with it.
That said, the better arguments are ones already made, notably these two:
1. Hope that Alan has finished the lightweight version of Armory not needing a full node.
2. Export your private keys one by one from the offline wallet, import them in another client, and move the money like that.
But let's address these...
I know that this is gonna happen sooner rather than later, more so if I'm looking at a 30 or 40 year timeframe.
Plus, be aware that this is a 66x size increase in just over 3 years, and we are reportedly still in the early stages.
66 times in 3 years for 40 years:
40/3 = 13.33
66*13.33 = 880
25gb * 880 =
22 TB40 years ago top of the line storage:
http://royal.pingdom.com/2012/09/04/hard-drive-market-1974/5MB could be rented, but we should find a purchase price.
This link is better:
https://en.wikipedia.org/wiki/History_of_IBM_magnetic_disk_drives#IBM_3330Purchase price:
http://www-03.ibm.com/ibm/history/exhibits/storage/storage_3330.html$87,000 for 5MB (and this is a whole system)...
$87000 / 5 = $17400/MB
Today we can pay $20 for 64GB, so how much per MB is that?
$20 / 64000 = $.0003125/MB
So technology has increased to a difference per megabyte per dollar of:
17400 / .0003125 = 55.68 million
Assume that technology growth sucks hardcore and we only see a 1 million technological difference over the next 40 years:
64GB * 1 million = 64 PB
That means in 40 years we can expect a 64PB drive to cost $20. That is still more than capable of carrying a 22TB blockchain. Even accounting for inflation, you're still talking at least a 1PB drive for $20 in 2054.
I mean, we can fiddle with these numbers all we'd like, but we have absolutely no idea what's in store even 5-10 years from now, let alone 30-40. Even assuming we have technological growth at 1/55th the rate we have the last 40 years, we're still well within reason of storing the blockchain on a $20 drive. And if you asked any scientist in the 1970s (because, keep in mind, we couldn't ask the average person who had little idea what computers even were) if it was likely that in 40 years we'd have 64gb drives available for 55 million times less per megabyte than we did in the 1970s, they'd probably laugh and say it might be better, but not THAT much better.
I agree with scalability concerns, but mostly on the network bandwidth front, not so much on the back-end storage aspect.