A good way to guess the numbers is look at your playstation trophy section. Only 33% of people who made an account ever even got to attaining 1 piece of exotic loot. So slash that 25 million by 2/3rds right there. How many of that 33% got a few exotics then quit? I'd say at least half, so you end up with 16% from 25 million. Then how many are multiple characters on an account? Are they counting each character? If so you can take another 2/3rds from the 16%. After a lot of guess work i'd say about 5% of 25 million is not far off the mark.