How does the bitcoin security tied on miners reward, this is not so, even after the entire bitcoin has been mined, miners will solely depend on the amount charged on transaction fees which I even see it's more better by then for them than now, whereby they may not have to mine bitcoin again and still make their earnings upon every block confirmation reward.
You actually do have a point and that's because it's not directly related to security but it is indirectly when you begin to actually consider selfish mining but in reality it's almost impossible. The problem , which is actually a good thing is that no one actually wants to take advantage of the 51% rule and that's because even when they have enough hash rate they'll consider it stupid.
Hash rates are very valuable in bitcoin mining especially with the rate of increase in difficulty and the current difficulty too.