The coin is predictable in every way that it needs to be in order to be a useful currency.
So, randomness, only for the sake of randomness, that happens to average out over time to be non-random like I was saying.
Not really, because the hash rate fluctuations are the main variable for the coin supply. You can predict the long-term current supply only if you can predict the hashrate for the period you are interested in. But just average hasrate prediction will not be enough though, you will need to guess the ratios of the different "supply rate" windows since the function is non-linear - e.g. the same AVERAGE hasrate for two different equally-sized periods will most surely result in different number of generated coins.