Op return stops arbitrary data at the point of submission but does nothing if someone already gets a miner to include it in a block - then consensus ensures all nodes have this data. Core argument is that it does not work as filtering (technically correct) and knots argument is that it does work at the point of entry so to speak but if you lift the limit you introduce unknown amounts of unknown spam that was likely filtered to begin with.
It’s a values / philosophical argument at this point. Node runners don’t want to facilitate this kind of spammy behavior but core says you run what we make - and if you don’t like it, run knots. So then people run knots and core and supporters are upset that people are running knots. Go figure …