Sybil attacking depends on the measures used by whoever it is who is paying for the account with their Steem. If you are getting your account from the steemit web site, they have various rules about social media accounts, IP addresses, etc. I'm sure they can all be bypassed, but that may or may not be worth it for $10 of value that has further restrictions on liquidating it (and these rules can all be changed if it becomes a problem; nothing at all is promised when it comes to free accounts). The minimum to power down is not the primary obstacle to milking the free accounts, but it does reduce the incentive somewhat, so other barriers can be a bit lower, inconveniencing legitimate users less.

The minimum is set by witness vote. Might decline, might increase. There is no set rule. It is possible the minimum might at some point not be sufficient to actually use the network much if at all, since bandwidth scarcity could increase the amount of SP needed. That depends on a number of things including usage, technology, etc.


In practice the main effect is to sprinkle a little rewards around to a lot of people. I doubt anyone will make a living as a professional commenter, but who knows.