The network effect is powerful and the only thing keeping Bitcoin on top. Sure someone could fork it and 'start from scratch'.... but everyone who has been following the project from early on including 100% of developers, partners, and speculators would have been mining. I will take my chances with the market.
I don't assume people will honor them as 'equal'.... in fact, they will not be equal because ProtoShares will be the foundation of many future AltShares where as BitShares will just be the currency exchange. If ProtoShares have any value at all it will be because people understand the network effect. When BitShares do come out they would have to bet that the social contract was too weak and everyone would adopt a new chain starting from 0 rather than honor the contract.... very dubious.
The "network effect" is from people telling their friends and relatives about something they think is cool and useful. Part of what made Bitcoin interesting and useful was the fact that everyone started on an equal footing. Everyone who read the features of Bitcoin and liked what they heard could go get them on an equal footing to anyone else. That fairness from the beginning is what made Bitcoin's networking effect so strong. Even if new people looking to mine wish they could have mined at a lower difficulty the people who had previously concluded that the mining was fair prevented them from forking.
If the features of Bitshares are not ready before mining starts people will fork the blockchain after the useful features are implemented. Mining should not start until the primary features of the currency are ready. At that point you should just release the protocol itself instead of an alt coin. I think you're making a big mistake releasing an alt coin before it has full intended functionality.