As you learn more about crypto and traditional finance, it'll be fun to compare the two. Your confusion about the role of an exchange in traditional finance might be because you see the crypto world, where a single entity often performs the roles that many entities perform in traditional finance (exchange, clearing firm, broker, etc.).
> I do think the exchanges in traditional finance shouldn't have required HFTs in the first place (i.e. it's an antiquated technology)
I'm not quite sure what this means, but it's important to understand that HFTs exist in the crypto space as well. Capital markets don't function particularly well without marker makers, and absent some rule explicitly preventing high speed trading, marker makers will tend towards being the fastest traders in any market.