How hard is it to design a PIN lock? There have been so many bugs of these nature in the last years, and probably before... And the only solution is to disable

How hard is it to design a PIN lock? There have been so many bugs of these nature in the last years, and probably before... And the only solution is to disable Siri, instead of having a "Disable Siri from the lockscreen and make the lockscreen just a fracking lockscreen." EDIT: I know, the simplest things end up not being so simple when you look at them closely and try to add a few features. Apparent simplicity in software design is often deceitful.