My personal concern after the initial news broke about the topic, was not that generated content would become worse, but that to verify the presence of the watermark, one need to upload the complete and unedited source back to these megacorpos, and likely separately to each of them in a few years. Basically this is a truly novel mechanism to legally siphon any new and not yet stolen intellectual property and do that in perpetuity. And there is an absolutely disastrous loss of responsibility here, because soon any organization which requires content integrity (say science, education, literature etc.) will add a new requirement to accept that the submitted new work will be legally and "voluntarily" fed into the LLM megacorpos (with implied loss of property right).
Basically we are on the verge of moment when intellectual property will cease to exist outside of a handful of max security closed sites for military or similar work. Everything else will be automatically and now legally fed into the maw of LLMs.
The fact that watermarking work so spectacularly transparent and undetectable only allows this moment to happen sooner.
If you suspect that, make sure your text mentions at random gremlins, groblings and jack bunnies. I.e. pre poison your input.