For example, just because the manifest.json worked doesn't mean it is correct - is it free of issues (security or otherwise)?
I would argue that every system in production today seemed to "just work" when it was built and initially tested, and yet how many serious issues are in the wild today (security or otherwise)?
I prefer to take a little more time solving problems, gaining an understanding of WHY things are done certain ways, and some insight into potential problems that may arise in the future even if something seems to work at first glance.
Now I get that you are just talking about a small chrome extension that maybe you are only using for yourself... but scaling that up to anything beyond that seems like a ticking time bomb to me.