Op, the trading fees is never specified neither is it stabled. If the self custodian blockchain fee increases, so also the centralized exchange fees also increases.
Right in the first place, you or minor investors should ask yourself a question that Why does bitcoineers have to pay much fees in a self custodian blockchain? This is actually because you're being billed to pay for an assured secured blockchain unlike the centralized exchange where you're not a whole in charge of your assets. On this mindset single handedly, neither you or the other investors who're below large institutions wouldn't prepare to store your assets on the centralized exchange just because of a lower fee.
What happens if your assets is compromised out of greeds of trying to skip high fees? That's just a food for thought!
And at this point of contact, bitcoin can never be succeeded to be centralized.

The Op mean to say if only a large institutions can afford to pay the high fees of a self custodian blockchain, then the minor bitcoin Investors may pull over to have their bitcoin assets stored in a centralized exchange which could encounter to a centralization of bitcoin since most Investors would have their assets there in centralized exchanges.