Forking a coin is free. Anyone can fork any coin at any time. Most miners will mine whatever makes money, but some may mine for a possible future payout or altruistic reasons.
A fork is generally considered successful if it carries economic value. That is decided by the amount of users transacting in the currency. Working exchanges are a requirement for this, but not enough (as has been experimentally demonstrated).
> The ETC fork would have been successful at any non-zero support level from people with the ability to mine.
It would be economically irrational for people to send transactions to a fork with a single miner since that miner could unilaterally rewrite any transactions as they see fit. This is why I think miners have the ability to control forking.