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.
I think that if 50% bitcoin will lost so it means that the supply will interrupted and therefore the price will even increase more and more, because those people who will be holding bitcoin, they may not be ready to sell their bitcoin and therefore we can expect a very big change and increase in the price of bitcoin.
What? I wasn't talking about the coins being lost but dumped on an exchange due to people abandoning the currency. That's what OP was asking about.
Do you honestly think that if 50% was dumped the price would increase due to lack of supply? For 50% to be dumped there would have to be both supply and demand, but that demand would go down since people would panic seeing the price is going down like crazy.