Leaving hazy judgement calls up to the police for enforcement, at least in the US, has not fared particularly well for non-white people. See, for example, the history of enforcement of "jaywalking", a "crime" which was recently decriminalized in California for precisely that reason. So I wouldn't advocate for a system where we just "let police officers make that call" regarding whether a driver is going at a safe speed. At least not in the US.

And "design speed" may be an interesting or useful thing to think about for roads that are far away from houses or business, like highways. But if you're near those things then it suddenly matters a lot less whether the speed is safe for the driver, and a lot more whether it's safe for the other important road users: pedestrians and bicyclists/other micro-mobility users. In those cases, speed limits absolutely make sense. And bizarrely, at least in the bay area, it seems like those are areas where speed enforcement (and other traffic enforcement) are practically non-existent.

So I have to disagree. I know you're talking about highways, but even there I'd rather have something closer to an objective standard being enforced, rather than the simple will of the police.