You just slightly estimated it. I don't know if it's on your realization, but it seems rather disappointing to me than (probably) to you. The price won't be that fluctuating forever; not that there's a proof for this, but over time, the supply will be steady and if the demand is too (global adoption), then the miners can only rely on the amount of the transactions.
Otherwise, the security will be slowly decreasing over time.
I definitely hope it won't fluctuate forever. Yes, I agree that the point about the fluctuation is completely valid. I'm making a point about having either an increase in the price or the increase in the adoption. Either of which increases the profitability, either by relation to the fees or the profit from mining.
Granted, I do think that I might not be right about a sustained security as there is a ton of factors on that but I don't think that the security (or rather the hashrate) of the network would ever dwindle to dangerous levels. If it ever gets to the point where it is better to be dishonest than to be honest, then that would be a problem because the system fails.
I guess he/she means in 2140. They'll reward 0.00000000 BTC, right? Just like OP_RETURNs.
No as in, there is a difference between a coinbase transaction and the coinbase rewards.