What's stopping us NOW, from re-riting the Blockchain from the genesis block, with 'new' valid blocks?
Math. You need stupidly large amounts of energy to do that, and there are immeasurably small chances to accomplish it otherwise.
I would be happy to try something like a proof of concept... on a private Testnet or something.......
Try writing more about it in detail. You have either forgotten something or you lack fundamental knowledge.
Changing the blockchain requires more nodes than the entire node network currently active.
Changing the blockchain requires just one attack, running just one node.