It's not a coincidence that racism was redefined to include the power element at exactly the same time black people became powerful. In a world where a white person can encounter a black teacher, a black principal, a black journalist, a black police officer, a black judge, all under a black President, the "power structure" argument for racism falls flat. Or, to put it another way, this is precisely why saying "white people" is racist because, by the definition you use, black people now have the power to be racist.

And, to be clear, the term "white people" is never used outside of a derogatory or stereotypical context. There is no doubt this is racist, and nothing exempts the user from being so.

The bottom line is, saying "white people" is offensive. Ask yourself why that isn't enough.