I think you've stopped, because you've realized that reasonably, it won't be secure in the far future; thing that you surely dislike, but you can't think of another possible scenario. Did I find it? Because, so do I. 
Don't make me get into doubt/scenarios cycle, it's not a nice one
Actually I've stopped because it was either deadlock, either things I cannot reasonably foresee. It can be bad, it can be fine; we don't know; too many things will evolve hugely in that many years...
@NeuroticFish Thats a good point. But what @ranochigo says is much more valid. If the block reward is "only" 6000$ per block, and a 1Million dollar transaction could be forged for 100.000$ worth of mining equiptment and electricity the transaction cannot be trusted. But maybe its a good thing, that in the future big transaction cannot be trusted.
Beware, you get into deadlock. If such a transaction cannot be trusted, big companies will not do them, don't you think? No.
Some miners will get out of business. Some others will enter. I expect that the ones who made their homework and will have their own power for free will not be few and will keep the network safe. If that won't happen, Bitcoin will be dead; with the amount of money in it, I don't think that the big companies will allow that to happen. (loops inside loops inside... it's not easy...)