... I already explained that we do not lose fifty percent of our security at the halving, since the exchange rate also effects this dynamic ...
Not really. This year, we are paying roughly 10% of Bitcoin's total value, or what people here call "market cap," to secure it. In other words, we are currently spending roughly a dime to store (secure) a dollar for a year.
After the halvening, we will be paying only a nickel.
It doesn't matter how much a dollar is worth, the ratio remains the same.
Same for efficiency/cost of mining gear.
What matter is how much a Bitcoin is worth compared to fiat, since if the market capitalization of Bitcoin increases, the block reward will be worth more in terms of fiat value and real world purchasing power. This would stimulate increased mining since there will be profit to be made, since miners costs are still predominately in Fiat and it is the purchasing power of those Bitcoins that really matters not the amount of Bitcoins that are made. This is why the ratio does not stay the same, if the market cap of Bitcoins doubles so does the amount payed for security, these two things are explicitly linked.
But it doesn't matter. A safe that's "just good enough" to secure 10 dollars is probably not good enough to secure a billion, agree?
The attacker doesn't want to break Bitcoin to prove a point, the amount he's willing to spend on the attack is directly proportional to the potential reward. So if it costs him a buck ten to steal a dollar now, after the halving it will cost him 55 cents.
Bitcoin price in fiat doesn't play into this, because it affects both the cost of the attack and the reward equally.
Tell me if I'm being unclear.
The cost of the attack is measured in fiat. Since to buy and setup more then fifty one percent of the hashpower would cost more then two hundred and sixty million dollars (just ran a rudimentary calculation). If the price of Bitcoin however went up, it would in effect mean that the Bitcoin protocol is paying more for its security in terms of fiat or real world purchasing power. This would incentivize more miners to come into the ecosystem thereby increasing its security. Therefore it can be argued if the price of Bitcoin doubles that the security of Bitcoin would also double meaning that it would cost twice as much to attack the network, in this case for example it would cost more then five hundred and twenty million dollars, and this is just for the setup costs, not including maintenance or even electricity costs which would over the long term might even cost more then the machines themselves. Not to mention that the attacker would lose much more from such an attack then they could possibly ever gain.
I guess I am being unclear.
We can
measure denominate the cost of attack in BTC, fiat or old hubcaps, it doesn't matter.
Think of yourself as a
burglar Swiper from Dora the Explorer.
You denominate your swiping effort in dollars. It currently costs you $1.10 of 'effort' to rob $1 from Dora's pocket (due to the security measures she has in place).
You're a pragmatic Swiper, swiping for profit rather than sport. After doing some quick back-of-envelope (also swiped) calculations, you realize it ain't worth it. You choose not to swipe.
But then Dora goes and cuts her security spending (in dollars) in half, and now it only costs you $0.55 to swipe a buck.
Wat do, Swiper?
Doe it matter if we replace "dollar" with BTC in this example?
P.S. To avoid complicated maths, model this at the limit, i.e. "the cost of mining 1 BTC is slightly less than market price of 1 BTC, as per satoshi's prognostication.
P.P.S: Which would you do:
1. spend 10 dollars to make five dollars
2. spend "five hundred and twenty million dollars" to make a billion dollars
(chose one)