Language evolves, the meaning of words change. It's a natural part of the evolution of language over time. You say hang up the phone or dial a phone number, don't you? But you don't literally hang up a phone when you're done with it or turn a literal dial anymore. But they used to work that way, so we still use that language even though the original meaning is lost.

I would argue that although this phrase originated from dismissing the opinions of actual boomers, it's become a convenient shorthand way of calling someone out of touch with current social trends without having to adapt new phrases every time a new generation takes the place of the boomers as being out of touch.