First of I think the highest price of bitcoin in the year 2017 is only 18 to 19k, not 20k while on this year, we are not sure yet if we will end this year with a price of 20k or not because there are still some months left and there's always a chance for the BTC price to change. In the event the price didn't recovers a lot, then what can we do?

You can always expect that each new year, BTC will also reach a new ATH. That's wrong as that's not how this market works but there are two cycles here which needs to be followed. The bear and bulls can occur at a random time I believe. That is why. BTC may lost its momentum for a while but it surely can come back much stronger next time so no, hodling and buying BTC is not pointless.