Exercise:
1) suppose ALL miners agree upon 4 MB blocks, even though all users want to have 1 MB blocks. The miners only make one chain, with 4 MB blocks. What are users and nodes, including exchanges, going to do ?
Emergency hard fork PoW change -> all miners are fired and all their gear is effectively useless.
2) suppose that ALL miners agree upon 1 MB blocks, even though all users want to have 4 MB blocks. The miners only make one chain, with 1 MB blocks. What are users and nodes, including exchanges, going to do ?
Same as above if needed.
Both situations are a hostile takeover.