seriously, if bitcoin is not massively used (like thousands of transactions per second) by the time block rewards become really small, it can pretty much be considered a failure.
Bitcoin will become more and more a store of value. Why is that considered to be a failure, when this is by design?
Because, if there are not enough transactions, the miners will stop mining, and the network becomes less secure, and more likely to be controlled by a single malicious person.
This is a true statement, however it is not looking like things will work out this way. Transaction growth has grown exponentially since the birth of bitcoin, even as the price has stagnated. As a result the miners are getting larger amounts of TX rewards as time progresses.
I also think the network security is greater then it really needs to be now. I would hypothesize that the difficulty could decrease by a large factor and the network would still be sufficiently secure.
well, normally yes. But if a lot of miners go down, just to suddenly reappear, that might be problematic. But only if a single entity holds all of those miners. Which is pretty unlikely to happen anyway. Even if old miners are given away for free, they'd still use a lot of power compared to the newer miners, so it'd still not be profitable to mine. Not sure though if it'd be profitable to use all those miners to attack the network, but probably not worth to even try it. It will cost millions if not billions in power alone, and the chances of failure are high. Even if you DO manage to pull of a 51% attack, chances are you'll just make all bitcoin worthless instantly, thus rendering your whole investment useless.
Also, for the record, someone mentioned energy efficiency in Watt/Ghash/second. That'd be Joule/Hash.