Or lets say if the price really goes high and when the year end comes, won't the people sell it before the decline arrives ?
For sure when the price arrives high again in sometime, many will start selling but we don't know on how long will stay and when decline will come.
If people start selling, don't you guys think that the decline will arrive much faster than the last year ?
We don't know, it can go faster or it won't or it will gradually decline. I hope that this year's increase and decrease is different from before and is going to be on a higher level of increase but lower level of decrease.
You should sell Bitcoin when the market value is more than its initial value. By this I mean to say you should always think about maximizing your profits and this can only happen when you sell your coins during the time when you feel to be in profit. They predict the prices of the coin to rise high and beyond $20000 in the end of the year and lets see the market response to the speculations. But a situation which makes well off should be utilized.