I think the benefits of open source in the hardware space aren't perfectly analogous to software. As you point out, due to the fixed and somewhat opaque nature of hardware, changing hardware yourself is far from trivial, what the Shenzhen ecosystem does allow for however is faster iteration within the market itself.

More steps of trial and error within shorter time, more things are being recycled, more people get their hands on designs and so forth. This doesn't always directly make things more customizable, but it does increase the pace at which the ecosystem moves.

The problem I see is that those iterations are constrained to not touch core parts of the hardware due to the lack of code and documentation being out there in public. Working with existing SOCs in different ways than their manufacturers ever thought possible is pretty common, many in the Linux-Sunxi community have pushed the envelope on how little power some SOCs can use, made possible in part by hardware features like a tiny, slow, power sipping RISC core that was on most Allwinner chips but was never leveraged before.