If I buy just 5 bitcoin today 2500$, Next 5 years bitcoin will be 15000$
this is the way that I should invest or not?
Well, for me it is good idea. You should buy bitcoins now until its price is not that big enough because there's a possibility that the every year will pass bitcoins price will goes up too. Furthermore, I do believe that if you invest your money on bitcoin and wait for the right time to sell you could guarantee that you will get a high profit.
5 bitcoins today is around 13000$, and in 5 years 5 bitcoins can worth 26000$ or 75000$, but who can guarantee that. I'm sure that here in forum there is many people who hold much more coins, and they hold for a long time and they don't plan to stop any time soon. Price will go up, and that is in what most of us here believe, if OP have 13000 on a side and he doesn't know where to invest it then he should buy bitcoins for his money.
Before we reach 4 digits price we will see many ups and downs, its like that with bitcoin, and I think bitcoin needs couple more years before we get closer to 4 digits.