The thing is, the one and only purpose of putting CSAM onto the chain would be to attack Bitcoin. There is no compelling case to be made that the blockchain or the relay network could be used as an uncensorable file sharing tool where in order to stop that, Bitcoin would need to be stopped. That's like forbidding all water pipes just because some sicko put poison into the water supply once.

The attack of putting CSAM on the blockchain doesn't get materially harder with the relay network not supporting 100kB OP_RETURN as the blockchain already supports it and the attacker could get it mined over time by simply submitting it directly to small enough miners that don't filter.

To re-iterate: Yes, it gets harder to submit that one picture of a crime but given what there is to be gained by destroying Bitcoin - if that was the downfall of Bitcoin - is many orders of magnitude more than what it would cost to put the material there, the difference is not material.