It's a natural progression once mobile devices became too numerous to overlook.
The first thing you need to understand: responsive design it's not to conditionally control resource download. Media queries are not conditional comments. Like the name says, they are queries. They establish a dialog between the web page and the browser and the device. Thing of it like providing dynamic CSS capabilities. And looking at dynamic HTML, I'm sure it's not bad nor will it go away soon.
So, it has nothing to do with serving lesser content or different version of the same page. It's simply about being responsive to a simple 90 degrees rotation of a mobile device, from portrait to landscape or vice-versa.
Even more, it's about providing you with a all-in-one flexible layout, that will make for a good browsing experience across a wide range of screen sizes, from as low as 240px to as high as 1900px or more.
A powerful concept, since, from experience, you want to serve just one CSS stylesheet per website. The same website, for all mobile and desktop. Adapting it self. Need more?
It became so talked about that some mistake it for buzzword. It's not. It's such a powerful concept it gained traction by allowing web developers to keep the pace with the technology advancement and not make their design look like something from 1990's.
Tapping the screen to zoom in to better see content doesn't help much the user when there is also a form to be filled up and to send to the server. Try it on mobile and you'll see a bad UX. Responsive design helps you exactly and precisely with that: user experience.
And finally, no, an old flexible or elastic layout it's not the same with responsive design. Those that relied on that understood their error the moment they tried it on a mobile device.
And you can't possibly understand what's so peculiar about responsive design until you seriously try to shake your old ways and start mobile first. Providing a 960px-and-up flexible or elastic design that degrades well just won't cut it.