Mining centralization is a negative consequence of not blowing open the bitcoin transaction relay op return data carrier did.

But is this the only way to ensure miners choose to take transactions from the mempool instead of out of band?

I’m thinking something like proof of propagation, which I don’t think can work, or disconnecting from peers that send you a nonstandard transaction…at least this way you’d have two bitcoin transaction relay networks: the standard network and various libre relay networks. The main point is to give all miners access to the same transactions without creating a reason for out of band payments.