The fact that transactions can currently be included in blocks without the consent of nodes is not due to Bitcoin itself, but to the centralized block creation by a few large mining pools.
In a truly decentralized mining system, where no one knows who will find the next block, this would not be possible without the agreement of the nodes.
This is why allowing OP_RETURN without limits is dangerous. Nodes would have to process and store far more data, which would push out smaller operators. Unequal mempools would emerge when large amounts of spam compete with real transactions at the same fee levels. Since many nodes can only hold a limited size (for example 200 MB) while the network may contain in the future far more data (for example 800 MB), each node would end up storing a different set of transactions. As a result, block propagation would slow down because nodes are no longer working from the same transaction base.
In the end, only large pools with expensive infrastructure would benefit. If we want to keep mining decentralized and move it further in that direction, we need filters and clear limits instead of allowing unlimited data spam in the network.