Designing any productivity or professional software with mobile first is ridiculous imho, computers are for doing work, mobile is for the plebs to consume content.
Well that's just silly.
I tend to agree that the vast majority business software is being used on a computer. The entire SaaS industry is web-based.
However, it all comes down to how people are using your software.
I like the way they put it in this Neilson Norman Group article:
"Mobile first not mobile only."