Because the goal isn't "keep this exact version of the app alive and running". The prototype is never the whole application. If your only metric is incidents, then yeah, don't ever touch the code again.

You can get a few feet closer to the moon by building a treehouse, but you still can't turn it into a spaceship.

The prototype is never the whole application.

In a world where people (stakeholders, Product, and dev teams alike) want the prototype to be the full set of MVP features, this is not true.