Your importance to the network is not just based on how much you hold.
It's based on a number of things and the balance is just one of them. One is how often you transact with others,
correct
another is if you keep open a reliable node...etc
wrong
The amount of harvested blocks are completely indepent from the fact if you run a reliable node or not. We do have a supernode program in the works though, but that has nothing to do with harvesting/POI:
http://blog.nem.io/supernodes/Supernodes are still in development, we are in an open beta phase at this time. So don't expect any rewards yet.
If you held millions of XEM and then did nothing with it, your importance to the network would quickly drop.
Since the balance is one of the parameters which affect your importance score, it will not "quickly drop" if you have millions. But it WILL decrease if you dont transfer, that is correct.
Mabe this isn't clear for you, too: There are no block rewards. You only earn the fees inside the block which you harvested ("mined" in Bitcoin language). So you can harvest a block and earn nothing, if the block was empty (no transactions happened inside that block).
New people, read this:
http://blog.nem.io/nem-tutorial-list/