Everything is used by criminals, not just Bitcoin. For sure, the great majority of the criminals are using fiat. The more dangerous ones are definitely using fiat. They are the ones who are wearing suits and are occupying high positions in the governments of different countries and in the top executive positions of various international companies. In the low level, even scissors and frying pans and pins are used by criminals as well.
That's true. Bitcoin is just one way to transact illegal transactions that's why we can say that it is used by criminals. But to be honest, there are so many ways to be used by criminals, but some of them just choose bitcoin.
Indeed it is true that transaction of bitcoin is now very fast and easy people will not have to wait for it, but it does not mean bitcoin will decrease the value, some people use bitcoin for good and some for bad so it is not like bitcoin is bad, people just take the advantages of it that bitcoin has anonymous nature and it is untraceable so it is easy for them to use it. Bitcoin is digital currency and it has a lot of advantages along with disadvantages.