The endlessly apathetic hypothetical "average" user can't see past next week's paycheck, let alone their long-term best software interests. We are the ones who have to look to the future and prepare for it now. Wanting access to source code of core infrastructure is part of that preparation, and "average" users will be indirect beneficiaries via our improved ability to write reliable software.
To add another perspective, constantly targeting "average" is a great way to stay mediocre (see regression to the mean).
Finally, "average" users will benefit immensely from those who are inspired by the ability to tinker at a young age.
You can never do that with proprietary software. If you want something changed or fixed, you must appeal to the singular entity with monopoly access to the source.
To most users Linux is just as much of a black box as Windows is.
Even among Linux fans, the number of people who actually need to customize the kernel is tiny. The number of people who can customize the kernel is even smaller.
So where's the freedom? At the OS level, all that's happened is that the lockdown has moved from corporations to a subset of the developer community.
Most end users aren't any more empowered than they used to be.
Now - it's different in the web and language spaces, where there's a steady simmer of framework development, and many popular web projects/products wouldn't have been possible without framework sharing.
But there's still plenty of proprietary content there. Just try to get Google or Facebook to share their data collections with you and see how politically relevant open source 'freedom' is then.
If that seems like a tangent, it's missing the point that the value of a system doesn't come from the source code - it comes from the system as a whole, and includes usability, community reach, innovation, invention, and data.
Open source pretends to be a huge lever for freedom, but it's more like a battered fork caught in an avalanche.
In computing, the world-changing leverage is elsewhere, and always has been.