Blockchain is also an address, a password, and there is not much difference from an account, if the frauds get the password, they can still embezzle it.
the security also has nothing to do with the blockchain. it comes from the cryptography that bitcoin uses.
One of the reasons we use cryptocurrency is that we can avoid the inflation, but both Bitcoin and Ether have succeeded in making hard forks several times. In addition, coins’ value is uncertain.
second, forks does not change anything about scarcity of bitcoin. they are poor copies of something good.
so far none of these 3 had anything to do with your title!
exchanges are third parties that are using the technology to offer some services. they don't define the ecosystem.