Non-developers don't care about the source.

I recall a motorcycle which people thought was nice and cheap, but had this major problem where the motor was baked into a single block in such a way that you could not repair most things in it.

Are most motorcyclists also mechanics? No. Do most motorcyclists care about the minor details of the inner parts of the motor, or scientific methods used within? No. Most just want a motorcycle that is cheap, function properly, and yet this specific motorcycle was strongly disliked and viewed as a trap. You could not repair it when it broke and you could not recover the investment by reselling it.

So say you are a non-developer and buying a e-reader with a bunch of DRM. The publisher go busts and you want to recover your investment. At that point you do care that you own a brick rather than some general purpose computer, but its to late. You are stuck in the trap.

A good chunk of my paycheck proves you wrong - we develop custom addons for an Open Source software for non-developers (individuals and companies).

In what way does a paycheck prove me wrong? I could just as easily argue that a good chunk of my paycheck proves that always bracing conditionals is something companies care about.

If bracing conditionals was indispensable for providing your services, I'd argue that they would.

In what way is that different for the end user compared to a closed source product with a public extension API?

We often do some profound changes to the system. This is Python, so you don't actually need to change the source, but being able to read it is indispensable. Plus, the license allows them to safely purchase our services and re-use the resulting system everywhere.

Even if you are not a developer, having the source makes it possible to hire someone else to accomplish a task.

Users cares about features, bugs, regressions. Who fixes these things does not matter, but if we don't have the source we can't fix it.