What happen if Miners stop Mining BTC???

If miners stop mining the network, than the security is degraded and the network would be susceptible to attacks such as the infamous %51 attack.

Today with the interest in Bitcoin, there isn't much of a risk about worrying about this. Even when block rewards halve over time, the fees are suppose to account for miners incentives. This of course could be up for debate, but in theory from the inception of Bitcoin, there should always be a miners incentive

Go ahead and buy for the longer term, keep them safe in a hardware wallet for years before selling them. Nothing will happen to the bitcoin network as miners will not stop mining as long as there is some incentive to them or being the fees when sending a bitcoin transaction. There will always be transaction fees as long as people will keep using bitcoins and by the calculation the last coin will be mined to make 21 mln coins available in the year 2140, which pretty sure we would all be dead by then so do not worry as nothing will happen, except that the price may skyrocket after those years and you be a rich man back then when you will sell your coins.

What we describing is a scenario where a portion of the miners stop hashing which makes the network more susceptible to such attacks. What OP is describing is a scenario in which all(?) miners stop mining Bitcoin. This is a very unusual and improbable case in which Bitcoin would most likely be going towards its death. However, let's assume that all of the turn on machines and nobody turns new machines on (also unlikely) the Bitcoin network would just come to a halt. Transactions would just go into mempools but they would never end up being confirmed. If you understand how Bitcoin functions, then you'd know that it couldn't work in such a scenario.

if the price getting lower even after halving happening the miners could possibly looking for another way to earn more profit rather than holding on bitcoin,just for example alt,but if they still see bitcoin profitable then why move,100$ will indeed make miners get away but even 300$ after halving will make miners thinking twice to continue the mining unless they got very cheap electricity.

The movement of chain will stop. Automatically all transaction will be unconfirmed. The Bitcoin economy will be in chaos. This will result to fear and the buy support will start to be pulled out. And then one by one cpu will start mining it again but due to global low hashrate, this will be susceptible to double spending. But then all those who are holding BTC will make sure this wont happen so they will start mining it by themselves and the domino effect follows. Basically mining Bitcoin wont end even if all coins are mined. This is one of the reason, personal interest and investment security. So be calm and hold your BTC for as long you want...