surely if we allow people to choose their own lock in time, everyone's just going to go for the minimum period?
No. It all depends on the right incentives. Would you be on Steem if you'd earn nothing? Most likely not.
Same goes for powering up. If there are no incentives, then people will go for the minimum required period, but given enough incentives, this is easily changed.
This is going to become a debate about how to weight the incentives for what period of lock-in isn't it?!?
Inevitable I guess!