Those are 2 different questions. They wouldn't even be the same people involved.

Your appsec engineers will be the ones bug hunting with Mythos or whatever. Your regular engineers will be the ones implementing features. If the feature is a bad design and functions badly as a result, that is entirely orthogonal to whether it contains coding errors or bugs.

Also, there are bugs on the integration side that won't be caught running Mythos against source code, because it's not a source code issue. Even using Mythos for DAST is going to be very limited compared to how actual users will move through something as complex as a 'mature' OS like Windows.