alt Hacker NewsPrivacy is always the most popular for some reason, but also the least consequential and relevant angle in the real life.
Watermarks are not "spymarks". They're DRM. I wouldn't worry about advertisers tracking conversions. I would worry about the "analog hole" being closed. Think of no longer being able to even photograph your phone screen, because pixels on it carry digital watermark that's robust enough to survive being photographed - I.e. the kind currently used to tag AI generated images - and then every phone and computer refusing to display resulting photo because the app disallowed capturing its pixels.
The actual problem in your scenario isn't the watermarks, it's people not being in control of their own devices. When that's the case you're already screwed. When it's not, the watermarks are just data you can ignore if you want to.