You might not be there target user. You can't please every user and some sites want to add features that look great / add value when viewed on a larger monitors.
Mobile first sites make the desktop experience bland and simplier than they need to be.
It is still the best solution to the variable screensizes reality but it does make the web a little blander then need be.
You don't have to choose one or the other, you can have a simpler (but usable) interface for mobile and a more feature-ful version for desktop. This isn't rocket surgery.
You are referring to functionality my comment was more related to look / feel.
If you go with a popular framework with the mobile first approach you must design with those limitations on your full desktop site as well. If you go with Bootstrap and use the grid layout both layouts will use grid.
I know this might be shocking but you can make a website without a framework. In fact, it probably would have worked out better for this site as it has such a simple design. HTML plus a handful of custom CSS rules would have sufficed and it wouldn't lock you into any "limitations". This is not a hard problem, not even close.
