Something complicated will happen for an indeterminate but probably fairly short period of time, and then one fork will emerge as a winner.
Nobody wants to buy unspendable BTC, so it's in almost everyone's interest to find consensus, even if it's tragedy-of-the-commons consensus, very quickly.
Nobody wants to buy unspendable BTC, so it's in almost everyone's interest to find consensus, even if it's tragedy-of-the-commons consensus, very quickly.