if they hadn't sold second hand then you're left with less people buying so say 9000 is the highest ever, and (one bit you keep missing) then less people continuing to play. when those that would normally sell second hand stop you're left with in this example 4000 less players, so now you have 5000 left, where as with second hand players you have 10000 still as those 4000 unused copies are still being played, therefore second hand sales will only increase the amount of players the continue to play. it's simple. the numbers may be smaller but it's so obvious that second hand sales will only result in more players over the life time of the game, not less.
edit: and one thing i complete forgot which the op pointed out, when you have less players that continue to play a game online you will sturggle to get new sales, so with second hand gamers keeping the number of players at 10k you'll get even more sales of new copies. i wouldn't suggest to someone to buy an online game that has few players
/sigh.
I'll try and put it simple for you.
You start with 9000 players. 3000 sell 2nd hand. Still keeps the numbers level at 9000. No increase, because there hasn't been more sales. Just pass downs to others on a 1:1 ratio!
If there were no 2nd hand market. Fair enough the game could possibly die unless there were more sales. Which we are not talking about.