* Now imagine in far future that the target is for the first time < 64219. So the hash searched has the shape 0000000000000000000000000000000000FFFF, the probability of collision is ultra-high.
I don't really like the sun image, because it is often used where a known 2^128 attack exists. But in this case it is appropriate. There is a big chance that we don't get there.
If we get that far, bitcoin has another problem. I think block hashes must all be different so if the target stays at 64219 then bitcoin will run out of valid block hashes after some time. Hmm, in that case fewer blocks will be found the difficulty will increase again, so maybe it repairs itself... but the difficulty adjustment will be quite bad in that scenario.
