What will happen once after a fork, every decides to leave/sell out their bitcoin for the forked alt. Whatever the reason, whichever the fork and outcome name, would that be the end of bitcoin? How will the btc value be afterwards?
And how would the miners react to it?
You've already answered your own question. If everybody sell their coins the value will be 0. I mean it wouldn't even be everyone as not all coins are available for trading. Some of them are on abandoned or lost wallets and so on, so even if only 50% of coins in circulation got dumped in a short time (like 1 month) the price would temporarily plummet to double digits. Exchanges wouldn't be able to hold liquidity.