It is correct that before Core v30, miners could also include large OP_RETURN transactions in blocks, but that choice was made by clearly identifiable actors. Miners are public-facing companies. Deliberately mining large OP_RETURN data could lead to reputational damage, loss of hashrate, and in extreme cases legal consequences. That acted as a natural social and economic brake.
With the opening of OP_RETURN, responsibility shifts from individual miners to the network as a whole. Every node now relays these transactions, regardless of their content. This removes clear attribution to a responsible actor. What used to be a conscious choice by a few miners becomes a structural property of the protocol.
Large OP_RETURN data and inscriptions increase storage, bandwidth, and computational requirements. That raises the cost of running a node. Over time, fewer people can afford to operate their own nodes, which weakens decentralization and concentrates influence among large operators.
In the long run, this can alter Bitcoin’s level of decentralization and change the balance of power within the network.