It’s already happening for years. Steps: 1) identify competitor blogs 2) scrape all their posts 3) Run it through nlp to rewrite words, phrasing and sentence structures while keeping content. 4) Tidy up using SEO guidelines, linking and keyword research. 5) publish 100s of articles a month as one single person

Actually GPT-3 detectors can be built https://www.technologyreview.com/2019/03/12/136668/an-ai-for...

But it'll detect human written one too. So expect many false positives.

Yes you are right there are already lots of apps which people use to change the content.