The world has not moved on. Only a small minority ever considered sex different than gender. That concept didn't gain popularity until very recently, within the last 10 years. And now that people are seeing where that misconception leads, many realize that the two words should never have meant something different. John Money's experiments into gender were complete failures, or at least succesful in proving his theories incorrect.

The concept of gender as "the sex that you feel inside" is regressive and obviously wrong. How can one feel like something they are not? You can say you feel like a cat, but cannot possibly know what a cat feels like. You can only feel like what you are. If you feel dysphoria, that you aren't truly a man when you were born a man, you are simply mistaken, suffering from a mental disorder. You are a man, so you feel like a man, even if it seems incongruent to your nature.

Furthermore it's regressive because the modern concept of gender as separate than sex involves the idea of gender as presenting as the opposite sex. Basically the belief that a man or woman has specific ways of being. The Neil Degrasse Tyson idea, that if you put on a dress, that increases your percentage of being a woman. It does not of course. There's no right way to be a woman. Second wave feminism promoted the idea that a woman can do typically male activities and jobs, wear pants, etc. So it follows that it's quite regressive to say you are less of a woman if you wear pants, and enjoy sports, hate cooking, etc...