Privacy 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.
We've long lost the part about control of our devices. We're fighting last futile battles to turn the tide back on mobile, and save PCs from extinction. But that's like a fraction of tech around us today, and everything else is already out of our control.
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.