City Lords can still secede themselves can't they? I haven't really checked to be honest. but like I said, if people are willing to play in a realm that centralises power to that extent, power to them. There are plenty of other realms where people will fight tooth and nail to try and ensure the King has a balance of power against him.
Can they? I'm pretty sure that's for dukes only.
As for the concentration of power, "all that evil requires to succeed is for good men to do nothing" (or something like that). In a lightweight game, concentrating power is, in most cases, much easier to do than to prevent. Hell, even decentralization mostly only happens when the ruler decides by himself that he wants to spread the power around...
