This has actually highlighted something that I have occasionally wondered but never got around to asking. As block rewards drop lower and lower, and indeed approach zero... who is going to bother mining? Without miners, who then verifies transactions? Perhaps at that point the distributed infrastructure exists simply to support the payment network? But... who owns it? Makes you wonder if at that point we come full circle with centralised control once again?
Isn't that where difficulty factors in? If enough people quit mining the difficulty should fall so low that eventually it would become profitable again.
Technology is going to improve also, mining should continue to get faster/cheaper.
And while I'm thinking about difficulty - has it been determined that this is the problem with why no Scrypt blocks are being found? If this is the case, maybe in the future if enhancements are made (and it's possible to do) some sort of difficulty check could be added so that if a specific algorithm doesn't have a block found after say, 100 blocks (about an hour), that the difficulty of that specific algorithm could be adjusted down by a small percentage every 100 blocks until one is found?