My understanding is that the existing players will always have the potential to be within the top 20% of the game.
My guess would be that players have a 20% chance, so to say, to get an All-Star or better potential at creation. Thereby, assigning 'All-Star' as the potential of existing players, one effectively puts them in the 'top 20%' of players.
However, due to the large amount of existing players, those top 20% will likely end up being more than 20% for starters, but will slowly be adjusted downwards. Unfortunately, there is probably no other way which is fair to everyone.
I am not sure where I stand on the issue, but this is surely a major change. One effect is that it will segment the market nicely: if you need a 12th man who should only provide a warm body in blowouts, you can buy a scrub without potential and be done with it.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."