Here's the difference with Core:

1) So I fire up 2000 nodes, hire a coder to mod the Core code to set 200MB as cap.

2) I make the miners belive this is the prefered block size.

3) They all go "yayyy tons of fees!" and hire a coder to mod the Core code so they can start producing 200MB block.

4) I fill up the blocks for the next 10 days

5) Bye bye 75% of all network nodes

6) I pretty much run the network

Not that you could actually do (2) anyway, but that's just me defending Core again. As I mentioned, this is just a bit more convenient in BU. Someone with the resources to spin out 1/3 of the total number of nodes now on the network, and mining pools that are actually mining the blocks now, are going to find it trivial to do (1) and (3) right now, in Bitcoin with Core, if they wanted to. The convenience of BU is not going to be of any noticeable help, and even if you think it is, that horse has left the stable - the miners convinced can just download BU. Under that view, all that was necessary to kill Bitcoin was to release BU. That can't be right.