They absolutely can widen sidewalks (in many cases the current sidewalks are far narrower than they used to be.) Plenty of streets way narrower than this. FDNY’s guidance isn’t based on anything but being too lazy to analyze in detail any real potential problems and taking the shortcut of “the status quo needs to stay the same”.

The intent with all painted sidewalk space is that it’s a footprint for future capital projects to build in concrete with a curb. The city just doesn’t have the money to do it because pouring a little concrete costs a billion dollars here thanks to corruption and graft.