From my experience it is not hard to make it a habit (e.g. using `::at()` and declaring variables as `auto const x = ...;` is muscle memory at this point) but the code becomes very verbose and looks like programmed defensively to a sometimes ridiculous extent.

So I understand your skepticism. To address the point of relying on developers being "ideal": just turn the compiler warning flags up to eleven, make all warnings errors, and run your tests two times - one time with sanitisers, and the other under Valgrind. This won't catch all errors, sure, but will still make you more confident in your code's reliability.