That is totally true, because if the central banks decide to start printing massive amounts of bills, we will lose purchasing power (because of the inflation that is caused by printing extra cash). So the value that we give to these established currencies is derived from the trust we put in central banks.

And exactly that is the reason that people are moving to crypto's, because for example for bitcoin, there is a specific amount of bitcoin that will ever be created through mining (21million). No one can change that not even satoshi (the developer of bitcoin). So here instead of just having to trust the central banks, we have a guarantee that there won't be created extra bitcoins after the 21million.