Is the statement that "we cannot abstract out our own perspective from our view of the external world" a fatal blow to realism?
Even if the statement were true, it would not speak "conclusively" to truth or falsity of an external world. The statement is not a metaphysical statement but an epistemological statement. It says "we cannot know if an external world exists", rather than "an external world does not exist". I do not believe it is possible to reach such a fundamental metaphysical conclusion based on this epistemological proposition.
I also believe that the statement is incorrect. I believe it is possible, in one very important sense, to abstract our own perspective from our view of the external world.
What we think we know about the world is not just taken from our own experience or perspective. Our knowledge of the world is built from the shared experiences of millions of people, written down and recorded over history. The process of scientific discovery is by its very nature the process of abstracting our own perspective from our view of the external world.
We don't just passively experience things in real-time through our five senses and then later recall these experiences via our memory. We are also blessed with the power of reason. I think that by our powers of reason we provide structure to the external world and that the only coherent explanation of why and how this works is that the external world must have structure and exist independently of us.
I think a realist approach is the only credible and coherent approach to take - I look upon rejections of realism as somewhat "trivial" attempts to twist logic and reason into reaching an absurd conclusion.
It is my view that the burden of proof lies with those that reject realism - is there any good reason to think that the external world does not exist? The subjectivity of secondary qualities is easily explained, and rather than casting doubt on the existence of things in themselves, it lends support to the common-sense realist approach that the external world does exist.