It's not so much double the design work, it's double the code maintenance.

I'm of two minds on this. I agree with your complaint that "mobile first" (or just responsiveness in general) has tended to reduce the pleasantness of the Desktop experience. As a web application developer, the idea of having to maintain two separate codebases - one for mobile and one for desktop - is a big "no thank-you." So responsiveness tends to win on maintenance overhead.