We have significant historical documentation and data that suggests his analysis is correct. No one is claiming that the original intent is different than what he's saying. Are you asserting that the original writers truly meant all people, men and women, and truly meant all forms of discrimination? No one else is asserting this, and therefore you would stand quite alone if you did.
What everyone else is saying is that we can and should interpret the constitution according to our current language usage and culture, not on the language and culture that existed at the time it was written.
Originalism has a sound, logical basis in historical data and understanding. It's not really the extreme that you suppose ("What are the thoughts of the writer...") it's merely interpreting the document with baseline of the time (culture, language) that it was created in.
The funny thing is that many people agree with originalism on certain issues. Take, for instance, the idea that corporations are very nearly "people" for many intents and purposes. The framers did not have this view in mind, and many people agree with our current quasi-person state that "incorporated" companies exist in simply because they do agree with the "original intent" of the document. It's a murky situation, because to become incorporated literally means to become a person. Given that they pay taxes, shouldn't they then have representation beyond lobbying - ie, voting? It's the reason corporations are allowed to advertise freely - they are actually held to the first amendment in that regard. Go to Germany and many other countries and corporations don't have such protections.
At any rate, originalism isn't absurd any more than saying that the constitution changes over time merely because our language and culture changes. I believe there's a balance to be struck between the two.