The engines of a 747 are much more complex and designing them takes many more man-hours of engineering than does designing comfortable and spacious seats.

But assuming the engines don't fail to perform correctly, which one do you think 99.9% of passengers care more about?

Engines aren't just "correct" or "incorrect", they perform differently. Users switched to Chrome from IE because Chrome was well marketed on a website they already used (google) and was way, way faster than IE. Many also switched on OS X because it was a bit faster, more feature-full, and marketed/perceived as way faster. It wasn't because they loved the Chrome UI.

I still regularly have to help "common people" find everything but the new-tab button in Chrome (and every browser).

Finally, it's clear that UI/UX designers always say they're making software "more intuitive" and "a joy to use" but no one (except grumpy engineers like me...) know what to click or where stuff is hidden anyway. It's all just UI churn, around in circles. Show feature prominently because users don't know how to find it, then hide it because the UI is too cluttered. Repeat.

EDIT: and all that's not the main point, sorry ... anyway, the "mass market" or "common" user's ignorance doesn't change the importance of the engine to the "open web". And don't forget how Firefox started, with only particularly savvy users caring to install it.

I agree -- I would totally fly on a plane with uncomfortable seats but an engine that would get me from Tokyo to Seattle 40% faster.

And I am not saying the renderer doesn't matter. My contention is just that the featureset and the UI probably matters more today, for most people. Historically, that wasn't true. (I consider switching from IE "historical". I don't know anybody who uses MSIE today, except for one-off reason like accessing some legacy banking system.)