Listen, Chicken Little, if you have 5% of the hash rate, your attempt to "split the network" will go nowhere. The Bitcoin.com pool has around that much of the hash rate, and their accidental 1 MB+ block had no effect on the network at all.

Also, if a large enough portion of the hash rate increases the max block size, the rest of the miners won't be left behind, they will jump on board.

You make it sound like miners have to choose between Core or BU, but they don't. They can use whatever software they want. They can change the rules whenever they want. They don't need the Core priesthood to bless their changes.

Finally, miners already have full control. They have the ultimate say. That is how Bitcoin works. That is how it is structured. They do the proof-of-work. They decide what is valid and what is not valid. Of course, the other full nodes have some leverage, and users have some economic influence. But in the end, whether you like it or not, miners decide.