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.
I agree here. Price could go that high that it would be much more expensive investment to buy 5 bitcoins 5 years from now, and as 'Barbut' said we can not know is it going to be 26 or 75 thousand in that moment. And it really depends on financial state of investor. For me, it's science fiction at this moment. I would need to have really good stash so I could decide to invest 12-13 thousand dollars in bitcoin for saving on 5 years.