M-dot sites aren't dead, just dying.
A recent report released by Pure Oxygen Labs surveyed the Internet Retailer 500 Guide (2014) and found use of m-dot sites for ecommerce dropped from 79% in 2013 to 59%, with dynamic serving and responsive sites increasing 12% and 15%, respectively, with share shifting away from first-generation mobile solutions even further this year.
Dynamic serving is currently used more often than responsive among m-commerce sites, expected to pass 20% in 2015. Though responsive has several advantages, retailers are opting for dynamic serving to avoid responsive's performance drag (a major factor in bounce, conversion rate and customer satisfaction).
Responsive vs Dynamic
The main difference between responsive and dynamic (also known as adaptive) is adaptive allows you to target device-specific HTML, rather than load a single HTML file that uses CSS and media queries to format the experience.
Aside from being faster, the advantages of adaptive are:
You don't need a complete website overhaul
Responsive projects typically require a redesign from the ground up, and thorough testing across devices to ensure breakpoints don't become break points. Rather than using CSS and media queries, the adaptive approach uses device-specific HTML that's pulled by the device (client) or detected by the server.
It's likely better than SEO
A website can keep to one-URL (vs a m.site copy) for each page with either responsive or adaptive, but a faster page load speed is likely an edge over responsive to search engines, who do use page load speed and bounce rates back to search pages as ranking factors.
There are always exceptions. A responsive site can be optimized for performance, and an adaptive site can load slowly due to page weight, number of HTML options to load, or server/technical issues.
You can design contextual experiences
While responsive design offers some ability to show and hide content to different screen sizes, laser-targeting to specific devices isn't possible.
You may want to show different content to Apple users vs. Android. For example, iOS8 has a horrid Price-is-Right feel to its menu selection. With adaptive, you could override the native selection tools for such devices to a custom method of input. If future OS versions correct the usability issue, you can still target that version to remaining iOS8 users.
Responsive wins back a point on being able to scale across devices without creating multiple layouts.
While both approaches can be labor intensive (vs. m.dot sites which can be done quick and on the cheap), dynamic/adaptive appears to the more popular choice of online retailers in 2015.