A key problem with Caplan’s trinity is that most of it is easily replaceable. Getting good grades at college does signal intelligence and conscientiousness, but these could be signaled far more easily and cheaply. It’s very easy to signal intelligence via test scores: IQ is surprisingly predictive of many other desirable cognitive traits. This need not require literal IQ tests—standardized tests like the SAT or GRE are highly correlated with intelligence. In other cases, companies use IQ-like tests (e.g. tech companies’ coding interviews). These are also significantly harder to cheat on than college courses.
Caplan acknowledges that college grades are far from the best way to signal intelligence; what he doesn’t discuss is that they’re even further from the best way to signal conscientiousness. If you asked people why they don’t just learn college material independently without paying for college, I expect that a common response would simply be “oh, I don’t have the discipline for that”. College provides external frameworks, timetables, local incentives, and social pressure for people who aren’t conscientious enough to learn without that.