I disagree. Of course designing mobile-first is considerably easier than trying to do it later. However, I would also say that designing mobile-first is actually easier in general, even if you never plan to target mobile specifically.

The idea is that you are designing for responsiveness. You are removing the worry about what should be on the left or the right or how wide things should be. You are focusing on what things are related, the relative importance of different things, and the content/interaction.