A good heuristic leads to a reduction in the overall cost of a decision (combining the cost of making the decision with the cost of the consequences if you get it wrong).
A heuristic like "it's risky to rent a car to a male under 25" saves a lot of cost in terms of making the decision (background checks, accurately assessing the potential renter's driving skills and attitude towards safety, etc.) and has minimal downside (you only lose a small fraction of potential customers) and so it's a good heuristic.
A heuristic like "a 26-year-old working a clerical job who makes novel statements about the fundamental nature of reality is probably wrong" does reduce the decision cost (you don't have to analyze their statements) but it has a huge downside if you're wrong (you miss out on important insights which allow a wide range of new technologies to be developed). So even though it's a generally accurate heuristic, the cost of false negatives means that it's not a good one.
I agree with you in principle, but the combination of the base rate for "26-year-olds redefining reality" being so low and the consequences being not nearly as dire as you make out mean I stand by my claim, at least for the case of heuristics on how to identify dangerous cults.
With regards to the Einstein bit, per my above comment I still think that skepticism of GR was perfectly rational right up until it got an empirical demonstration. And it's not like the consequences for disbelieving Einstein prior to 1919 were that dire: the people who embraced relativity before then didn't see any major benefit for doing so, nor did it hurt society all that much (there was no technology between 1915 and 1919 that could've taken advantage of it).
Pascal's Wager (https://en.wikipedia.org/wiki/Pascal's_Wager) is also about a small but likely downside with a potentially large but unlikely upside. Do you think it's analogous to your 2nd case? If not, how is it different?
The difference is that in Pascal's Wager, the proposition is not a priori falsifiable, and so you cannot assign a reasonable expected cost (ie. taking probability into account) to either decision.
In the case of a 26-year-old making testable assertions about the nature of spacetime (right down to the assertion that space and time are interconnected), there's a known (if potentially large) cost to testing the assertions.
A heuristic like "it's risky to rent a car to a male under 25" saves a lot of cost in terms of making the decision (background checks, accurately assessing the potential renter's driving skills and attitude towards safety, etc.) and has minimal downside (you only lose a small fraction of potential customers) and so it's a good heuristic.
A heuristic like "a 26-year-old working a clerical job who makes novel statements about the fundamental nature of reality is probably wrong" does reduce the decision cost (you don't have to analyze their statements) but it has a huge downside if you're wrong (you miss out on important insights which allow a wide range of new technologies to be developed). So even though it's a generally accurate heuristic, the cost of false negatives means that it's not a good one.