Tell that to the agencies who supply me with 3 or 4 psds every week that their graphic designers have created. 
They don’t have the time, expertise or inclination to design in the browser which is why they hire me to do the dirty work.
Not everyone works the same way and the approach that you take usually reflects the way that your work is organised. If you are a designer and designing in the browsers then mobile first is fine. I have no issue with mobile first but just prefer the other way around because that’s the display I get presented to me. The code is much the same either way not to really matter. I code the PSD in a day and then move on to the next. No Less, no Sass, no problem.