Besides, would you really want this to be a prevalent feature on smartphones?
Yes. With virtually every other form of electronic communication we have today, we know that it can be (and likely is being) recorded and archived. E-mail, SMS, Facebook messages, video camera footage (without audio), license plate location details through ALPR, credit card usage information, utility usage information through smart meters... heck, even WHO you call and for how long is being recorded by the phone company and shared with law enforcement, no warrant required, since that's "just metadata" recorded by a "pen register" and doesn't fall under wiretap laws.
Now, ask yourself, who benefits from that fact that it's difficult for the average citizen to record their phone calls? And why is it that every entity with any significant power or revenue records their phone calls, and simply informs you of that fact without offering any choice? How exactly does it benefit me that all my interactions with Comcast or AT&T are recorded and kept by them and I have nothing?
Maybe you think there should be more protections for all the other forms of communication and data, but that's not going to happen. And just like it was in the interest of law enforcement for them to be able to record you, but not for you to be able to record them, I think you need to think hard about whether you really want to preserve an illusion of privacy, or actually be able level the playing field.