Tab, you brought up a good point about how philosophical inference/deduction suffers from an increasing margin of error. This is true. This is the real problem that philosophy faces: derviving A is fine but then B, C and D and… becomes more difficult. But difficult does not mean impossible, it just means difficult. There are two main ways to stabilize our speculative reason against this limit you mention: one is to continually check it against scientific discoveries and the other is to continually check it against itself, which means in the realm of our self-experience and world-experience.
A good philosopher always continuously tests and juxtaposes his ideas against each other, against his experiences, and against scientific discovery. The METHOD of philosophy is essentially different from that of science: philosophy introspects, which means that philosophy poses problems before the mind, in thought, and reconciles those problems in that same domain. From the purely scientific empirical perspective you are at least seeming to adhere to, such introspection is considered almost worthless, but in fact we can arrive at untold discoveries and insights simply by thinking. Even in the scientific method you need thought processes to make sense of the data that is observed and collected.
Existentialism and phenomenology have indeed given us very much, but what these and “pure philosophy” give is more intangible than things like computers and iPhones-- intangibles like a basis capacity for rational thought, logical positing, balancing our emotions against each other, moral sanity, aesthetic vision and values, and the whole range of developed subjectivity and more comprehensive, stabilized personality. The entire “self” is a philosophical construction first and foremost.
You shouldn’t underestimate the degree to which human subjectivity and “self” has changed over human history. Each major world-religion reflects a certain stage in this development but again, it’s intangible. If you’re only interested in tangible objects you can hold and see with your eyes then you’ll miss all this other progress that has been going on the so many thousands of years.
For example, without Greek culture and philosophy there would be no Christianity, and Christianity is just an outward representation of a new kind of human subjectivity-consciousness. I can refer you to more specific and elaborated theory on this if you want, just let me know… the bottom line is that subjectivity or “the self”, our self-experience, isn’t simply a given or fixed thing produced by the brain, but is in fact a complex entity evolving over time and formed largely by the experiences and ideas we are exposed to. No theory of mind can be complete without a detailed reckoning of the myriad ways in which individual brains interact with the extant history of accumulated ideas, social norms, and text that brains encounter after they are born and as they grow up through childhood and into adulthood.
The real “work” of human being is out there in the accumulated shared history of meaning, concepts, writing, affective norms and standards that we all encounter as we grow through childhood. The brain is a system for recording and responding to all of these things; but without them you simply have feral children, and no true humanity.
Precisely what we take as most given and which goes most unnoticed, namely our self and our subjective experience, is the primary thing being created and evolving over human history. This is why Being is invisible to itself along the dimension of its most necessary self and laws, at a certain point Dasein cannot apprehend itself quite simply because it is this “itself” already-always. A contradiction ensues along two fronts: 1) knowledge of oneself creates more self, leading to a forever-receding horizon line of Dasein, and 2) that which we most are, and would therefore most aim to bring into awareness, is precisely the most difficult to apprehend.
Of course scientific empirical knowledge doesn’t suffer from this kind of limit, since it is purely externalized knowledge. This is why science progresses so much faster in an overt sense, also why science doesn’t achieve very much in the way of actually shaping the development of human being over time. Christianity will continue to exert far more influence on the development of the self and subjectivity than will cognitive neuroscience, for example.