The real concern would be adding the watermark to the real thing, to let it slip through the cracks. However, not only would this be computationally expensive if it was properly implemented,

It wouldn't be expensive, you could do it on a laptop in a few seconds.

Unless, of course, we decide only large corporations should be allowed to generate images and completely outlaw all of the open source / free image generation software - that's not going to happen.

Most images are created with a "diffusion" model where you take an image, and run an algorithm that slightly modifies it. Over and over and over until you get what you want. You don't have to (and commonly don't - for the best results) start with a blank image. And you can run just a single pass, with the output being almost indistinguishable from the input.

This is a hard problem to solve and I think catching abuse after it happens is increasingly going to be more difficult. Better to focus on stopping the abuse from happening in the first place. E.g. by flagging and investigating questionable behaviour by kids in schools. That approach is proven and works well.