Learning here... So pardon my ignorance.
If the argument is that spammers are adversarial and their activities damage the network with fragmented utxo's, then why would we suspect they would just use op-return? If they're adversarial and looking to damage the network, then leaving a door open for them doesn't mean they'll stop breaking windows.
And second thing I'm not clear on, one of the major arguments I heard is that this is a policy change and not a consensus change. That's true, but in the same sentence, it's often said that nodes are irrelevant unless they're "economic" nodes.
If nodes are irrelevant and only economic nodes matter, then why remove a policy that is configurable to the user anyway? Economic nodes can just change their own policies to allow the bloated op-return and never look back.
From that perspective it seems that the move is a "soft" consensus change. Removing user configuration options forces a change of network behavior without changing consensus.