Bitcoin is unstable but as you can see, the prices always go up no matter how many times it has been dropped.
No. There is no guarantee for that. For example, the exchange rates touched a peak of $1,230+ in 2013 December. Almost four years have passed, and we are yet to witness exchange rates at those levels (although now it is getting quite close). Have you imagined the situation of those who purchased Bitcoins back in 2013, for $1,230 per coin?