The "Supply and Demand" principle can simply explain this. Since no one is buying now then I'm pretty sure the price will become stagnant and if we assume also that all are selling their remaining holdings then this btc's price will continue dropping and will not recover (since we are not buying anymore).
Will bitcoin be as usable as now Or will it be a step towards the end?
Definitely not because this means that btc are now valueless for us. Tendency, we will prefer using another coin and this coin will have the chance to replaced btc and become on top.