Simple phrase for the business model of 'secretly' worsening terms of service?
A coverall phrase is sharp practice
Sharp practice or sharp dealing is a pejorative phrase to describe sneaky or cunning behavior that is technically within the rules of the law but borders on being unethical. https://en.wikipedia.org/wiki/Sharp_practice
Apart from that, I think stealth increases and stealth downgrades might serve. I can't think of a pithy phrase to include both terms except perhaps stealth degradation of service.