It is true that that's the standard originalist response.
The problem is that originalism attempts to make lawyers into historians, and they're bad at it. In honest, well-meaning attempts to be 'originalist', judges make mistakes[1]. In degenerate cases, originalism is simply an argument from authority to excuse cherry-picking.
> The problem is that originalism attempts to make lawyers into historians
So does textualism, and while lots of people accuse judges of having different philosophies, most (in the US; there are other judicial Philosophy that exist) of them (at least, in terms of how they justify decisions) are originalist, textualist, or somewhere in between.
Most of the complaints really are not about originalism vs. other philosophies, it's about people with a particular ideological bias (primarily, right-wing) in how they tend to see original intent accusing people who see the intent differently of not being originalist (left-leaning observers do also
complain about right-leaning “originalist” judges acting in bad faith, but don't tend to frame it as being about originalism as a virtue and deviation as a vice.) I mean, none of the people on the right praising originalism and raging against judges for not being sufficiently originalist were upset at Scalia, who explicitly and firmly rejected originalism in favor of strict textualism.
That's exactly it, by using very old precedents you essentially make the law random due to a range of conflicting cases none of which map 1:1. Often 'is X legal' is only decidable by a court case, that's not the rule of law that's effectively amending the rules after the fact.
This is pretty much the basis of the common law system. We write laws defining general principles, and then let the courts sort out the specifics. We don't really know what the law is until we have some court cases about it. The court cases use precedent to keep things consistent.
The alternative is to have really, really detailed laws, like in civil law systems that don't use stare decisis.
The problem is that originalism attempts to make lawyers into historians, and they're bad at it. In honest, well-meaning attempts to be 'originalist', judges make mistakes[1]. In degenerate cases, originalism is simply an argument from authority to excuse cherry-picking.
[1] One paper examining this theme: http://www.law.virginia.edu/pdf/f14_jurisprudence_symposium/...