What if miners aren't selling all their mined coins now? If they are only converting just enough coins to cover their fiat running costs, then they will convert exactly the same amount of coins to fiat because running costs will not suddenly jump after halving. So, neutral effect on price
Further, you don't take into account that halving means that the miners' revenue will also be halved. But revenue is
not profit, so, after the halving, they may actually begin suffering
losses...
What will they do, and what will they do even before what you think they will do?