Tell me what will the price be by the end of today? How about by the end of this week or month? You can't, can you?

Similarly you can't really speculate on Bitcoin's security regarding miners. When bitcoin started, miners were earning 50 bitcoins per block but that was barely worth 50 bucks. Today they are earning 3 bitcoins per block (3.125 to be exact) but it is worth $300,000.

My point is that just like people back in 2009-2010 could neither foresee nor believe some day miners would be earning $300k with 3BTC reward, we can't really predict what things are going to be like 10 years from now when reward is smaller or 100 years from now when the reward is zero.

This is a problem for the future to solve because it depends on a lot of things. I personally believe by that time bitcoin has either will have ceased to exist or significantly changed and will not be anything like it is today.