Bitcointalk is older than most of those exchange you call legit, I can't verify if the currency exchange board was a board on Bitcointalk from the very beginning but I know most exchange were carried out on the forum before those centralized exchange even thought of coming into existence.

The reason we have exchange board here on forum, was for member to exchange their coins easily since there was no exchange for that but now it involved to bypass KYC and exchange bitcoin P2P easily. Most of the exchange you hear of will run out of business in 5-10years time but Bitcointalk will still be around as far as bitcoin still has value that means the currency exchange board will still be around and forum member can use it to exchange their coins.