The problem is the whole idea of working from first principles to try to discover the actual truth, rather than just going along with other ideas that other people have created through history. The benefit of going along with other peoples' ideas is that they've been tested out for hundreds or thousands of years and been found to not be that destructive.

If you start reasoning from first principles you can very easily make an incorrect assumption somewhere down the line that snowballs until you're talking about shrimp welfare, or making decisions based on the potential sextillion humans that exist in 1,000,000 AD, or planning to convert the entire universe into unthinking organic matter that can only experience bliss.