Overall, I agree it's a problem, but I see responsive design as a (perhaps poor) solution to the convergence of three more fundamental problems.
The first being screen size fragmentation. Most tablets fall into a similar size/scale category, so you can have buffers (kinda like how you were saying). However, there are still corner cases where (e.g.) a tablet in landscape mode happens to trigger a different size category. The fact that resolution helps in fingerprinting users is pretty indicative that fragmentation is a huge problem.
The second being the lack of a good way for a page to know exactly what type of device it's displayed on. There are hints/etc. to get close enough. However, it's still a fundamental truth of web development that the browser can always lie to you. Not necessarily a bad thing, but a design challenge regardless.
The third is less of a technical or even a design problem, but I personally think it's the worst. More and more, companies are making increasingly complex sites with no real thought to user implications. I believe this will always be true in a web where the real customers are ad companies and search engines and the user is the product.
On the lines of another point you've made, I'm not suggesting we give up; there are still plenty of ways we could and should improve designs and frameworks (you've outlined some really good ones).
The first being screen size fragmentation. Most tablets fall into a similar size/scale category, so you can have buffers (kinda like how you were saying). However, there are still corner cases where (e.g.) a tablet in landscape mode happens to trigger a different size category. The fact that resolution helps in fingerprinting users is pretty indicative that fragmentation is a huge problem.
The second being the lack of a good way for a page to know exactly what type of device it's displayed on. There are hints/etc. to get close enough. However, it's still a fundamental truth of web development that the browser can always lie to you. Not necessarily a bad thing, but a design challenge regardless.
The third is less of a technical or even a design problem, but I personally think it's the worst. More and more, companies are making increasingly complex sites with no real thought to user implications. I believe this will always be true in a web where the real customers are ad companies and search engines and the user is the product.
On the lines of another point you've made, I'm not suggesting we give up; there are still plenty of ways we could and should improve designs and frameworks (you've outlined some really good ones).