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?
No it shouldn't be the way to invest in bitcoin.
yes bitcoin price may reach 10K, 15K or even more but you shouldn't only be thinking about how much "fiat" you gain from investing in bitcoin. that is never going to end well. instead learn more about what bitcoin is.
bit coins is a currency not something you put your money in for a short time to get rich. try using bitcoin to buy some stuff, try to learn more and see what it really offers. i assure you that you won't regret it.
and then the profit is just icing on the cake.
Not everyone will share pro-bitcoin attitude, for some people it is still pretty stupid internet money which simply happens to be growing very fast.
These newbies don't plan to use bitcoin as money, they buy it baited with bitcoin's store value properties.
In short - not every bitcoin holder needs to be in love with the idea of creating an alternative economy based on cryptography.