There is now a feature that scans every photo you receive. There previously was not something scanning every photo you receive. What used to be a conversation between 2 people is now a conversation between 2 people and an Apple blackbox (assuming you turn the feature on).
As with most of these types of tech, the proponents of the "this involves erosion of privacy" aren't necessarily concerned with the exact implementation as described on release, but with how the blackbox works and how the blackbox (and the laws/regulations/obligations around it) will change in the future, and the inability to change the settings of the blackbox, effectively letting Apple become the touchstone for what is 'appropriate', even if it doesn't align with your view of 'appropriate'.