1) Things go out of stock too much if it's a little more specific. 2) Prices are very sporadic and unstable. A subscription should be relatively predictable - with the exception of the obvious price inflation over time, but I'm talking month-to-month.
When you subscribe to something it doesn't "lock in the price" - prices go up and down month-to-month.
On the amazon side, there seems to be a bit of flex built in, when they'll still tell you the item is in stock, and tell you the price before the re-buy kicks in.
A lot of the items I "get told are unavailable, would I like to go with the alternative?" are still available, just for a higher price.
i.e. I think a lot of the "annoyance" is actually amazon bulk negotiating on your behalf. If say P&G decide to raise their price on their detergent, amazon has the power to decide whether 90% of their customers swallow the increase, or they all find an alternate brand. My guess is that amazon decides not to provide P&G detergent to a few hundred thousand subscribers in a month, that helps 'focus' the P&G rep in getting the price back down the next month.
Always thought it would be quite nifty if you could make a site like this for third-party services. I might see my 2 audible credits on there, and decide I've got enough, or need more. Or maybe apple could offer a free month - so I might pause my netflix. Could even combine stuff (free $10 Dominos credit added for each month of Netflix after month 12 etc).
Whole current system of every service having their own UI, is just a pain. Worked out paying for stuff works better then you can just stick it all on the same card - so why not manage it all from one place as well?