1) "having everything trading for essentially real money doesn't change that underlying level of engagement." It has been long established that this is absolutely the case. A classic example of this would be Diablo 3's RMAH effectively destroying the game by having the entire economy be driven by real-world profit-seeking grind
2) I consider this to be very misleading. All of these intra-game features are not made available by the blockchain itself, but by the details of the various Smart Contracts involved. In that sense, the set of instructions available to would-be developers to interact with a game is going to be limited by the operations (and the various rules governing them) that are present in the smart contracts. If a game features permission-less development like this, it's because its developers have decided to make the requisite operations available and documented. This is no different than a regular documented public API.
3) I see nothing preventing this from being done without the blockchain.
4) This is the one potentially interesting wrinkle, but I find it to have marginal interest at best. Assets are inherently immortal unless explicit steps are taken by developers to prevent them to be. What is actually at play here is Immortal Entity Ownership, aka Immortal Scarcity. That's very interesting from a revenue-making potential, but not really much more than that.
5) That does not make the game itself better though.