I recently retired from being production manager at a TV station. We were using iPhones to record fantastic quality video for years. I’ve yet to meet a professional who doesn’t think iPhone video or photos isn’t good enough. The one’s I’ve met and worked with love them, using them more and more. Now, you can’t change lenses, there are limitations, but for 75% of any professional work, they’re fantastic. I’ve told folks I shot on higher end cameras, they love the video, we never told them it was actually an iPhone. Folks can’t tell the difference, professional camera folks can in upper end productions. But there is no one-size-fits-all camera yet. iPhone video is great, but it still isn’t RED.
Now, if you’re working network TV or feature film, there’s LOTS of shots an iPhone won’t get for you, and RED cameras and professional studios have NEVER been on any sort of decline. Not sure who told you that, they’re wrong.