Doesn't this argument assume that bitcoin will be a medium of exchange in the future? If bitcoin remains as nothing more than a store of value, the volume of transactions would be substantially reduced, which would lead to low security? And if bitcoin has low security, then it's basically worthless.
So does that mean whether or not bitcoin survives depends on whether or not it becomes a medium of exchange? In other words, bitcoin being a widely adopted medium of exchange is a necessary condition for it to survive.
Depends. I don't foresee it being much more desirable to attack Bitcoin, even if the hashrate of the network drops significantly. The profits gained from an attack is far lower than the cost and the complexity of its execution and double spent Bitcoins can potentially just be worthless still. If it remains as a store of value but the value is still fairly high, then there isn't any problems.
There are tons of factors to Bitcoin's survival. If the block rewards were to be reduced exponentially right now, it won't survive. There isn't any time for the market and the miners to adapt to the far lower profits. If we were to stretch out the decrease in the block rewards, there is far lesser shock for the miners to bear. Important to note that the fees are starting to form a more significant part of the miner's income. If no one uses Bitcoin, then I wouldn't think that there is a point to keep Bitcoin afloat anyways.
As of now, blocks has consistently been filled, less some blocks in the recent weeks. Thus, it isn't a stretch to assume that if the capacity were to be somehow increased, then the transactions and subsequently the fees would be greater as well.