100% CDF would mean "with this amount of work you are absolutely certain to find a block". But that is never true. Unlike a lottery where you would perhaps be able to buy all the tickets, you can't do that in bitcoin.
As time approaches infinity, CDF approaches 100%. But just like time never reaches infinity, CDF can never be 100%.
Makes sense. But what about the case in which I'm brute forcing a private key. And my machine is the only one working on the problem. At some point in time a million bajillion years from now I would expect to have been able to try all possible combinations. Now assuming there is only one matching private key, it would be possible to find that key on the very last sequence tried. What is the CDF? Would 50% CDF be half that time, in this case?
Thanks for your explanations. Very lucid.