We have a lot of ideas for how to manage this. But we won't get to decide. We can put forth our solution and people will be free to use it or not. Over time, this will probably need to evolve.
I've learned to be less quick about saying something especially something I don't fully understand can't work. But I'm having a hard time reasoning about how the points your making result in a secure system.
Bitcoin's consensus algorithm is secure assuming that information is hard to stifle (the longest chain can't be hidden from you) and so long as conspiring dishonest parties do not control more than a (near-)majority of the hashpower.
If these assumptions are violated a high hashpower attackers can block and reverse transactions and replace their own transactions at a depth related to their hashpower share, but can do nothing else can't create inflation, can't steal funds from unrelated transactions, etc.
Is there a similar compact and fairly comprehensive expression of Ripple's security assumptions that could help people reason about the system?