Yes, it is foolish to save money according to the inflation of the country's money because it reduces the value of our money. so making the investment will be a very good decision for us. Although investment is very risky, if we can accept this risk and invest in the right place, we can make huge profits and save ourselves from inflation. so I personally think that investment from savings is the best idea to maintain the value of money as well as earn money.
Why you say "foolish" to people who want to secure their reserved money for the future?
It is not a bad decision to have saving, people can have reserved money for emergency or urgent needs in the future. When it is for saving purpose, I also keep it in our currency. It can be easier to use it at any time, we can pay anything with our fiats. But if all our funds to put on the investment, the risk will be too high. There is no guarantee to succeed in every investment, there is a chance to fail as well. Having both saving and investment is a wise way to deal with the risks. Saving enables us to have reserved money to use at any time, meanwhile investment let's us to multiply our money. So, I think smart people won't focus on investment only, they will try to have both of them (saving & investment).