I was reminded today of how in design the concept of the "fold" and the materiality of breakpoints still exerts a powerful influence over people.
The viewports.fyi presents the results of one studio's research into what happens in the real-world. Viewports skew smaller but their dimensions are all over the place.
Fluid responsive design is the utopian but how often does this happen in regular businesses? The advantage of the breakpoints concept is that you can do a few wireframes or high-fidelity screens and call it a day.
Shifting to a truly fluid layout is going to be a lot of work initially but then works for the largest audience.