In case my previous reference to the need for telepathy wasn’t clear, originalism is not based on what people wrote down. That’s textualism.
What the authors of the Constitution generally wrote on the subject was that they opposed the idea of original intent.
But activist conservative judges don’t worry about the text. They claim to have special insights into the minds of the founding fathers. Amazingly, the founding fathers were all thinking exactly like 21st century Republicans.
As with any religion, you start with the conclusions you desire and then work backwards to justify them. Any other modes of thinking are heresy.
Formal logical propositions must start with a truth. If they are not a truth, any answer, whether true, false, or meaningless, may emerge even following strict formalities. That the thoughts of the founders are known and applicable to modern law is not a truth. Therefore, any desired answer may be produced, especially if strict logic is thrown out the window. The Ninth Amendment plays no part in opposing this process.
There’s a lot to be said for a stable constitution, but the US is unique in the path belief in the “sanctity” of the constitution has taken. Much like “states rights”, it’s a lazy argument that’s been used so much lots of people believe it, even though, if you scratch the surface, it’s rarely used with any sort of consistency or adherence to principle.
Part of it is probably due to the US political system being extremely conservative (in the ‘all change is slow’ sense) and the judicial branch having used reinterpretation of the constitution as a way to “fix” bad precedence where other countries are more likely to be able to write new law.