Here’s some solid pragmatism from James Archer in Why is it so Easy to Get “Mobile First” Wrong?:

Mobile-first design isn’t, in itself, the right answer. It’s just an efficient answer. If you can design for one device and then tweak it to more or less function on other devices, you shave a lot of time off the design process. With the cornucopia of possible devices you’d otherwise have to design for, that’s definitely a good thing.