You mean many investors will cash out from bitcoin to altcoin after halving? I am not sure who got it wrong but I think the bitcoin price grows dramatically after the halving period. I don't think there will be any exception this time. So if you still want to invest in bitcoin then this is the time to jump in. Also altcoin project still in an affordable price and if bitcoin price remains stable we can see some altcoin shoot upward. So this can be your last chance to join bitcoin and altcoin both.
What do you think makes a coin looks bullish if not one factor of the Demand and Supply, whereas Supply goes beyond the demand then we face the bearish part of the market. This is where the halving comes in cutting the supply by two then we have the demand becoming much greater than the supply.
Most investors pulls out profits from there Bitcoin investment and purchase some Altcoins for their holding which will help skyrocket the price due to the demand, it becomes profitable to select wisely into any Altcoin now the prices are still low.